Modern chemistry

GIFT OF

Th'E TEMPLE PRIMERS

MODERN CHEMISTRY
Theoretical

By
WILLIAM RAMSAY, D.Sc.

THE HON. ROBERT BOYLE

CHEMISTRY

THEORETICAL

RAmSAY-DS?

19008K 29&UO BEDFORD-STaEET^UQPOKZN

rights reserved

MODERN CHEMISTRY

FIRST PART
THEORETICAL CHEMISTRY

CHAPTER
INTRODUCTORY

Elements — Phlogiston — Discovery of Oxygen — Com=
bining Proportions— Dalton's Laws— Gay* L us =
sac's Law of Volumes — Avogadro's Hypothesis —
A tomic Weigh ts — M olecular Weigh ts — D ulong
and Pet it's Law — Equivalents — Isomorphism.

ONE of the earliest questions asked by an intelligent child
is : " What is this made of? " " What is that made of? "
And the answer is generally more or less satisfactory.
For example, if the question relates to butter, the reply
may be, " From milk." It may be explained, besides,
that when milk is beaten up, or churned, the butter sepa-
rates, leaving skim-milk behind. But the question has not
been answered. The child may ask, " Was the butter in
the milk before it was churned ? or has it been made out
of the milk by the churning?" Possibly the person to
whom the question is addressed may know that the milk
contained the butter in the state of fine globules, and that
the process of churning b:eaks up the globules, and causes
them to stick together. The original question has not really
been answered ; and indeed it is not an easy one to reply to.
Precisely such questions suggested themselves to the people
of old, and they led to many speculations.

4

: .CHEMISTRY

.^Qn& of .these speculations was that things
whichl\v»e see'ar"olihd"uk wer.^ ,b\iihu,p out of elements, just
as a word is built up out of letters. Indeed, the word
elemens, which is the Latin word for element, is pro-
bably derived from the letters /, m, and «, and involves
that idea. The ancient Greeks surmised that there were
four such elements — earth, water, air, and fire. But as it
was obvious that some things, for instance gold and silver,
did not contain either water or air, the word element was
often used to signify, not the constituent of a thing, but
rather a property of a thing ; and it might have been said
that gold partook of the properties of earth and water,
because, like earth, it is not altered by being heated, and
yet it can take a fluid form like water if heated hot enough.
Hence the old word " element" had a double meaning;
it was sometimes used in the sense of " constituent," and
sometimes more in the sense of " property."

If a child is given a mechanical toy, his wish to see how
it works generally leads to his taking it to bits. This is
unfortunately only too easy ; but it is seldom that he
succeeds in putting it together again. Now, if we inquire
what a piece of wood or stone is made of, we can, after a
fashion, take them to bits ; we may pull the wood into
fibres, or we may crush the stone, and pick out the pieces
that appear to differ from each other in colour, if they are
large enough. But the fibres have much the same appear-
ance as the piece of wood, and the fragments of stone,
though somewhat different from each other, are still pieces
of stone. The question is still to be answered, of what do
wood and stone consist ? It is evident that some plan must
be tried by which the wood and stone will be unbuilt, as it
were, and by which they will yield their constituents.

It had long been noticed that many things are greatly
changed when heated. A piece of wood takes fire and
burns ; some kinds of stone melt ; some metals, such as lead
and iron, change into earthy-coloured powders. Surely
these changes ought to lead to some knowledge of the

PHLOGISTON 3

nature of wood, stone, and metals. It was long, however,
before it was recognised that the presence or absence of
air made a difference in the result of heating substances.
When attention was drawn to this difference, a new sug-
gestion was adopted. It was, that things, besides consist-
ing of or sharing the properties of earth, water, air, and
fire, also consist of, or at least are like, salt, sulphur, and
mercury. Salt dissolves when put into water ; so do many
other things. These things must either contain a kind of
salt to account for this property; or they must at least
share the property of salt, in so far as they dissolve. Simi-
larly, other things, especially metals, must either contain or
share the property of mercury, seeing that they shine with
the same kind of lustre ; and many things resemble sulphur
in so far as they burn and produce a smell in burning. And
it was often imagined that when things burn, the sulphur which
they contain flies away and disappears, just as ordinary sul-
phur, when set on fire, burns away completely, leaving nothing
behind. About the middle of the seventeenth century,
Johann Joachim Becher, a German alchemist, altered
somewhat the conception that substances contain, or are
like, salt, sulphur, and mercury ; he imagined all things
existing on the surface of the globe to contain three earths,
namely the mercurial, the glassy, and the fatty, the last
implying the property of being able to burn. And in.
the early years of the eighteenth century, Becher's pupil,
George Ernest Stahl, who was Professor of Medicine in
Jena, and later in Halle, two small German towns, made
an important addition to the ancient theories, namely, that
it was possible to restore the " sulphur," or the " fatty
earth," as Becher called it, to things which had been
deprived of it by burning, by heating them with other
substances rich in that constituent:

Phlogiston. — Stahl devised a new name for this com-
bustible constituent of substances, in order better to direct at-
tention to his new idea ; he called it " phlogiston," a word
which may be translated "burnableness," for it is derived

4 MODERN CHEMISTRY

from a Greek word signifying flame. Thus lead, which
when heated in air changes to an earthy dross, or, as it
was then termed, "calx," may be restored to the state of
metal by heating it with charcoal powder, or with flour,
or with any substance rich in " phlogiston ; " that is, with
any substance which is itself capable of burning. He sup-
posed that the lead was rich in " phlogiston ; " that when
it changed to lead-dross, the "phlogiston" escaped; but
that on heating with charcoal, the latter parted with its
phlogiston to the lead-dross, changing it back again into
lead. It is evident that this idea accounts for some of the
facts observed ; and it gained ground rapidly. But it had
been shown by the French physician Jean Rey, by the Eng-
lish philosopher Robert Boyle, and others, that metals, when
they changed into earthy powders on heating, gained weight;
and it is at least curious that the lead, on losing one of its
constituents, namely "phlogiston," should gain weight: one
would have expected that weight would be lost, not gained.
The way out of this difficulty was ingenious. We know, it
was said, that weight is due to the attraction of the earth ;
now, it is not impossible that the earth may repel phlo-
giston, instead of attracting it; and in that case a body would
grow heavier, instead of lighter, if it parts with phlogiston.
Another objection to the theory was that a candle, for
example, which is rich in phlogiston, goes out when made
to burn under a glass shade ; that is, when air is excluded.
True, said the phlogistonists; that is because the phlogiston
cannot escape. And because this theory gave a plausible
explanation of the common phenomenon of combustion, it
was widely accepted, and survived until the end of the
eighteenth century.

The idea had been steadily gaining ground that know-
ledge was to be acquired only by trial and failure. Francis
Bacon, Lord Verulam, at the end of the sixteenth cen-
tury wrote: "The true kind of experience is not the
mere groping of a man in the dark, who feels at random
to find his way, instead of waiting for the dawn or striking

DISCOVERY OF GASES 5

a light. ... It begins with an ordered — not chaotic —
arrangement of facts, deduces axioms from these, and from
the axioms again designs new experiments." Many ex-
periments were made on the action of heat on various
things, either alone or mixed with others. Boyle, who
introduced the word " analysis " to denote the art of dis-
covering one substance in presence of another, and who
contended for the use of the word "element" in the mean-
ing of a constituent of, and not as a property of matter, made
many such experiments, and systematically put Bacon's ad-
vice into practice. And so knowledge of facts gradually
accumulated, and speculations acquired some substantial
basis. To Priestley, a nonconformist clergyman of Bir-
mingham, we owe the discovery of numerous gases, and
Scheele, his contemporary, a Swedish apothecary, also en-
riched chemistry in this respect. The discovery of oxygen,
in 1774? was made simultaneously by both of these illus-
trious men. It had been generally supposed that all gases,
or, as they were then termed, "airs," were merely modifi-
cations of atmospheric air ; and it was not uncommonly
held that air, in consequence of its want of substantiality,
was but one remove from nothing. Joseph Black, Pro-
fessor of Chemistry in Edinburgh in the middle of the
eighteenth century, was the first to prepare carbonic acid
gas, or, as he termed it, "fixed air," in a pure, state ; and
by determining the loss of weight on heating its compound
with magnesia, to show that it was due to the escape of the
gas ; for he succeeded also in absorbing the gas, and re-
constituting the carbonate of magnesia, which then possessed
practically the same weight as it originally had. In spite
of this discovery, made in 1756, the doctrine was still gene-
rally held that burning substances lost their constituent
principle, "phlogiston; " and we owe to the French che-
mist Lavoisier the true explanation of the phenomenon
of combustion. Lavoisier had been informed by Priestley
in the autumn of 1774 of his discovery of what, accord-
ing to the views then current, he termed " dephlogisticated

6 MODERN CHEMISTRY

air ; " he proceeded to repeat an experiment which had
previously been made by Boyle, in heating metallic tin to
redness in a sealed glass vessel ; there was neither gain nor
. loss of weight, although the tin had been partly converted
into " calx ; " but on admitting air, he observed a gain in
weight, nearly equal to that which the tin had gained on
being calcined. The conclusion was obvious, that the gain
in weight was due to the absorption of a portion of the air
by the hot tin ; and he subsequently showed that the gain
was to be ascribed to the absorption of Priestley's " de-
phlogisticated air," of which Priestley had shown common
air to contain about one-fifth. And in 1777 Lavoisier
published the statements: —

1 i ) Substances burn only in pure air.

(2) This air is consumed in the combustion, and
the increase in weight of the substance burned is
equivalent to the decrease in weight of the air.

(3) The combustible body is, as a rule, converted
into an acid by its combination with the pure air,
but the metals, on the other hand, are converted into
" calces."

Oxygen. — This last statement explains the name which
he gave to Priestley's and Scheele's gas, namely oxygen, a
word derived from two Greek words, signifying " acid-
producer." The compounds of this substance he termed
"oxides;" and it is to him that we owe the system of
nomenclature now generally in use. Before the end of the
century, the doctrines of Lavoisier had gained almost
universal acceptance.

The word " analysis," as has been stated, was suggested
by Boyle, to signify the ascertaining the composition of
substances. Attempts were made by him, and by other
chemists, especially by Black, to perform quantitative
analyses during the seventeenth and the first half of the
eighteenth centuries. Priestley and Scheele tried to find the
relative proportions of oxygen in air with partial success ;

COMBINING PROPORTIONS 7

but it was not until Lavoisier had convinced most chemists
that oxygen was a substance, and not the negation of one,
like the absence of phlogiston, that serious attention was
directed to accurate determinations of quantity. And
towards the end of the eighteenth century fairly trustworthy
data began to accumulate.

Combining Proportions. — It became evident, chiefly
owing to the work of two German chemists, Wenzel and
Richter, that when an acid, such as vitriol or vinegar, is
mixed with a base, such as potash, and neutralised, as the
expression runs — that is, rendered incapable of changing the
colour of certain vegetable extracts and deprived of its sharp
taste — the same weight of base was always required to
neutralise the same weight of acid. And other examples of
apparently constant proportions between the constituents of
substances had also been observed. But the processes of
analysis were very imperfect, and the results by no means
always concordant; and there was some ground for the
statement made by Count Berthollet, a contemporary of
Lavoisier, in his Researches on the Laws of Affinity , published
in 1803, that the composition of chemical compounds was
variable, and not constant ; that, in fact, it depended on
circumstances, such as the proportions of the substances
present, on the temperature, on whether the substance pro-
duced was an insoluble solid, and so on. Berthollet's
statement was disputed by his countryman Proust, who, by
fairly accurate analyses, carried out during eight years of
controversy, proved the truth of the doctrine of constant
proportions. But in the course of his work, he found that
in certain cases two elements form more than one compound
with each other ; for example, tin combines with oxygen in
two proportions, each of them fixed and constant ; and iron
forms similarly two compounds with sulphur. Perhaps the
most exact experiments which had at that time been made
were those due to the Hon. Henry Cavendish, who having
discovered that water was composed of oxygen in union with
another gas, to which the name " hydrogen " was subse-

8 MODERN CHEMISTRY

quently given, determined the proportion of these constituents
with very great accuracy. He found that two volumes of
hydrogen invariably combine with one volume of oxygen to
produce water, neither hydrogen nor oxygen being left over.
Owing, however, to the method of expressing the composi-
tion of compounds, no relation was evident between the
proportions of the constituents. Thus Proust expressed the
results of his determination of the composition of the two
oxides of tin and of copper in parts per hundred : —

Suboxide of Protoxide of Suboxide of Oxide of
copper. copper. tin. tin.

Metal . . . 86.2 80 87 78.4

Oxygen . . 13.8 20 13 21.6

1 00.0 100 100 IOO.O

Had he calculated by simple proportion how much oxygen
is combined with the same weight of copper and tin in each
case, he would have found that the ratio of the oxygen in
the suboxide of copper to that in the protoxide is as 13.8 to
21.5 ; and in the two oxides of tin as 13 to 24. The
correct figures are : —

Suboxide of Protoxide of Suboxide of Oxide of
copper. copper. tin. tin.

Metal . . . 88.8 79.9 88.2 78.9

Oxygen . .11.2 20.1 11.8 21.1

IOO.O IOO.O IOO.O IOO.O

The ratio should be as I to 2 in each case ; and the fact
that Proust did not remark this is to be ascribed partly to his
method of stating his results, and partly to the inaccuracy of
his analyses.

Attention was first drawn to the existence of simple
proportionality between the amounts of one element forming
more than one compound with another by John^Dalton, a
Manchester schoolmaster, in 1 802 and the next succeeding

DALTON'S LAWS 9

years. In the year named, he described experiments " On
the proportion of the several gases in the atmosphere ; " and
he then stated: "The elements of oxygen may combine
with a certain portion of nitrous gas, or with twice that
portion, but with no intermediate quantity." And he later
illustrated the same fact by considering the composition of
two compounds of carbon with hydrogen, marsh gas, and
defiant gas, the former of which contains twice as much
hydrogen as the latter, proportionally to the same weight
of carbon.

The laws relating to the proportions in which various
elements combine are therefore usually called Dalton s
Laws ; they are : —

Dal ton's Laws. — The law of definite proportions : —
When two or more elemer^, combine with each other
to form a compound, thc,y combine in constant pro-
portions by weight.

The law of multiple proportions : — When two elements
form more than one compound with each other, they
combine in simple multiple proportions.

Thus, if A and B are definite weights of two elements,
the proportions in which they combine will be A with B ;
or A with 2B ; or A with 36 ; or 2A with B ; or 2A
with 3^ ; or 3 A with 2B, &c.

But Dalton not merely stated these facts ; he devised a
theory with a view to explaining them ; he revived and
gave defmiteness to the ancient conception that all substances
which we see around us consist of atoms. This idea is at
least as ancient as 400 B.C., and is to be found in the writ-
ings of the Greek philosophers. The theory, in the form
which Dalton gave it, is as follows: All compounds con-
sist of atoms of elements united with each other. An atom
is an indivisible (literally " uncuttable " ) particle, or,
more correctly, a particle which resists division. Each
atom has its own definite weight ; but as there is no apparent
means of determining this weight (for atoms are inconceiv-
ably small), we must be contented in determining their

io MODERN CHEMISTRY

weights relatively to each other. This we can do by
ascertaining the proportion in which they exist in their
compounds. Thus, knowing that water consists of oxygen
in combination with hydrogen, if the smallest particle of
water consists of one atom of each element, the relative
weights of the atoms will be found by discovering the
proportions by weight in which these elements are combined
with each other. Now, it is found that 8 parts by weight
of oxygen and I part of hydrogen by weight combine to
form 9 parts by weight of water ; hence an atom of oxygen
is eight times as heavy as an atom of hydrogen ; and an
atom of water is nine times as heavy.

We must beware of confusing this theory with the facts
on which it is founded ; indeed, Dalton' s contemporaries,
while accepting the facts, refused in many cases to accept
his theory. Sir Humphry Davy used the word " propor-
tion" in place of the word "atom ; " and Dr. Wollaston
preferred the word "equivalent." And even granting the
existence of atoms, the problem of determining their relative
weights is not so simple as would at first sight appear. For
how is it possible to know which of several compounds is
the one containing only one atom of each element ? The
two compounds of carbon with hydrogen by means of
which Dalton illustrated his law will furnish a good example
of this difficulty. While one of them, marsh-gas, consists
of one part by weight of hydrogen in combination with
three parts of carbon, the other consists of one part of
hydrogen, in union with six parts of carbon. Which of
these two contains one atom of each element ? If the
former, then the atom of carbon is three times as heavy as
the atom of hydrogen ; if the latter, it is six times. Dalton
was quite aware of this difficulty, but could devise no means
of overcoming it, and the numbers which he adopted were
only provisional.

The difficulty was solved chiefly by the experimental
work of Joseph Louis Gay-Lussac, Professor of Chemistry
in the Ecole Polytechnique in Paris, and by P. L. Dulong

GAY-LUSSAC'S LAW TI

and T. A. Petit, Director of, and Professor in, the same
school. The attention of Gay-Lussac having been directed
by the celebrated explorer Humboldt to the fact that water
is formed by the union of one volume of oxygen with two
volumes of hydrogen gas, he followed it up by the discovery
that other gases unite in very simple proportions by volume.
Of this we shall see many instances hereafter.

Gay=Lussac's Law of Volumes. — Stated in the
form of a law, Gay-Lussac's discovery was : — The weights
of equal volumes of both simple and compound gases
are proportional to their combining weights (or, to
use Dalton' s term, their atomic weights), or to rational
multiples of the latter. This law appeared as if it ought
to have some simple relation to Dalton's laws ; but there
is an apparent difficulty in reconciling them, which was
surmounted in 1 8 1 1 by Amadeo Avogadro, Professor of
Physics in Turin. The difficulty is this : —

Imagine a given volume, say a cubic inch, to be filled
with oxygen ; suppose it to contain a very large but un-
known number of atoms of oxygen, which we will call «.
This oxygen, if mixed with twice its volume of hydrogen,
or two cubic inches, and made to combine with it (which
can be done by heating the mixture with an electric
spark), yields nothing but water ; and neither of the gases
remains uncombined in excess. Let us suppose that n atoms
of oxygen combine with 2« atoms of hydrogen ; and as
water also, according to Dalton, consists of atoms, there will
be n atoms of water formed by their union. But experi-
ment shows that the water, in the state of water-gas or
steam, has a volume equal to that of the hydrogen from
which it was formed ; that is, n atoms of water-gas inhabit
a volume equal to that inhabited by ^n atoms of hydrogen.
From this it would appear that equal volumes of gases
do not contain equal numbers of atoms ; and while some
chemists supposed, with Dalton, that water consists of one
atom of oxygen in union with one atom of hydrogen, others
imagined that two atoms of hydrogen were present for each

12 MODERN CHEMISTRY

atom of oxygen, basing their conclusions on the fact that
two volumes of hydrogen combine with one volume of
oxygen, and considering it probable that equal volumes of
gases contain equal numbers of atoms. This last proba-
bility was maintained by Avogadro, and he defended his
doctrine by the following suggestion.

Avogadro9 s Hypothesis. — Substances consist of two
kinds of particles, each of which has been termed an atom
by Dalton. But they are in reality different. The smallest
particle, or, as Avogadro named it, molecule, of water, con-
sists of three atoms, two of hydrogen and one of oxygen.
But hydrogen gas and oxygen gas also consist of molecules,
each of them containing two atoms. The act of union of
these elements is to be regarded not as a case of combination
of atoms, as Dalton supposed, but rather as an exchange of
partners ; the atom of oxygen leaving the other atom with
which it had been combined, and uniting with two atoms
of hydrogen, each of which had similarly left its partner.
Thus n molecules of oxygen exchange partners with 2n
molecules of hydrogen, and form 2« molecules of water-gas ;
but whereas the molecules of oxygen and hydrogen each
contain two atoms, those of water-gas contain three. And
this explanation is consistent with the facts ; for while in
molecules of hydrogen and n molecules of oxygen, containing
together 6n atoms, react to form 2n molecules of water-
gas, the latter also contains 6n atoms, for each molecule of
water-gas contains three atoms.

Using the symbols H and O for one atom of hydrogen
and one atom of oxygen respectively, Dalton's idea of the
combination was —

H + O-HO.

On the assumption that equal volumes of the gases contain
equal numbers of atoms, the equation becomes —

Lastly, on Avogadro' s hypothesis, that the action is one

DULONG AND PETIT'S LAW 13

between molecules of hydrogen and molecules of oxygen,
each containing two atoms, the equation is : —

Granting Avogadro's hypothesis, the relative weights
of the atoms can be ascertained. For, as oxygen is
1 6 times as heavy as hydrogen, and as equal volumes of
these gases contain equal numbers of molecules, and more-
over as each molecule consists of two atoms, it follows that
an atom of oxygen is 16 times as heavy as an atom of
hydrogen.

Atomic Weight. — This is usually expressed by the
phrase — the atomic weight of oxygen is 16; for the
atomic weight of hydrogen, being the smallest known, was
taken as the unit.

The relative weight of a molecule can also be calculated;
as an instance, let us calculate the molecular weight of
water-gas. Experiment shows that a given volume of
water-gas is 9 times as heavy as an equal volume of hydro-
gen at the same temperature and pressure ; again, equal
volumes of gases contain equal numbers of molecules ;
therefore a molecule of water-gas is 9 times as heavy as
a molecule of hydrogen. But a molecule of hydrogen
consists of two atoms ; consequently a molecule of water-
gas is 1 8 times as heavy as an atom of hydrogen.

Molecular Weights. — This is usually expressed by-
saying that the molecular weight of water is 1 8 ; and inas-
much as it consists of two atoms of hydrogen in union with
one atom of oxygen, the weight of a molecule of water-gas
is equal to the sum of the weights of the atoms composing
it; for, (2 x i) + 16= 18.

Dulong and Petit' 's Law. — Let us now consider
the discovery of Dulong and Petit, already alluded to. In
1819 they made the announcement that the atoms of
simple substances, or elements, have equal capacity for heat.
It must be explained that equal weights of different sub-
stances require different amounts of heat to raise them

i4 MODERN CHEMISTRY

through the same interval of temperature. Thus, if the
amount of heat required to raise the temperature of a gram
of water from, let us say, o° C. to 100° C. be taken as
unity, it is found that only one-ninth of that amount is
required to raise an equal weight of iron through the
same range of temperature. Or, in other words, while
the specific heat of water is I, that of iron is -i, or in
decimals, 0.112. The quantity of heat necessary to be
imparted to one gram of water to raise its temperature
through i° C. is termed a heat-unit, or calory; but
sometimes the unit is chosen one hundred times as large,
and represents the heat required for a rise of temperature
from o° to 1 00°; and the French make use of a unit
one thousand times that of the smallest unit. Dulong
and Petit's^ discovery was, that if weights of the solid
elements be taken proportional to their atomic weights,
equal amounts of heat must be imparted to them in order
to raise them through the same interval of temperature.
The following table illustrates this fact, and exhibits some
of the results obtained by Dulong and Petit: —

Element. Atomic weight. Specific heat. Atomic heat.

Bismuth . . . 208 0.0288 6.0

Lead . . . 207 0.0293 6.0

Gold . . . 197 0.0298 5.8

Platinum . . 195 0.0314 6.1

Silver . . . 108 0.0570 6.1

Copper ... 63 0.0952 6.0

Iron .... 56 0.1138 6.4

Sulphur ... 32 0.1776 5.7

If the specific heat be taken as the heat required to raise
the temperature of one gram of each of these substances
through one degree, compared with that required for one
gram of water, the atomic heats of bismuth, lead, gold, and
the others represent the heats required for 208 grams of
bismuth, 207 grams of lead, and so on. It is evident
that they are all nearly equal. It should follow that the

EQUIVALENTS 15

specific heat of solid hydrogen must be also 6, since the
atomic weight is taken as I.

These facts, though clearly indicating the numbers which
should be taken for the atomic weights of the elements,
were neglected, until renewed attention was called to them
in 1858 by Cannizzaro, still Professor of Chemistry at
Rome. He pointed out that all that can be gained from
the analysis of a compound, for example an oxide, is the
"equivalent" of the element. And as an element, such
as iron, often forms more than one compound with other
elements, let us say oxygen or chlorine, it therefore may
possess more than one equivalent. But granting the atomic
hypothesis, its atom can have only one definite weight.
That atomic weight may, however, be inferred from its
specific heat, or from the density of its gaseous compounds.
Let us consider a concrete instance of each of these
methods.

The analysis of two of the oxides of iron leads to the
following results : —

Ferrous oxide. Ferric oxide.

Iron 77-77 70.00

Oxygen . . . 22.22 30.00

99.99 100.00

Equivalent. — Now, I gram of hydrogen combines
with 8 grams of oxygen in water ; and 8 is therefore
chosen as the equivalent of oxygen ; for the definition of
an equivalent is that amount of an element which will
combine with or replace one part by weight of
hydrogen. In ferrous oxide, since 22.22 grams of
oxygen combine with 77.77 grams of iron, 8 grams of

77.77 X 8

oxygen will combine with =28 grams of iron;

and in ferric oxide, 8 grams of oxygen are in combination

with =18.66 grams of iron. Thus the equivalent

j

16 MODERN CHEMISTRY

of iron in ferrous oxide is 28, and in ferric oxide 18.66.
The question now arises, What; is the atomic weight of
iron ? We have seen that the specific heat of iron is
0.1138; and we know that the specific heat of solid
hydrogen is probably 6. And as the specific heats of
elements are inversely as their atomic weights, we have
the proportion —

Specific heat Specific heat Atomic weight Atomic weight

of iron. of hydrogen. of hydrogen. of iron.

o.i 138 : 6 : : i : 52.7

The number 52.7 is nearly 2 x 28, and nearly 3 x 18.66 ;
these products give 56 ; and it must be remembered that
Dulong and Petit's law is not absolute, but merely an
approximation ; hence 56 is accepted as the true atomic
weight of iron.

The element sulphur forms a compound with hydrogen
which has the following composition : —

Sulphur 94.12 per cent.

Hydrogen 5.87 „

100.00

The gas is 17.1 times as heavy as hydrogen; and this
means that a molecule of hydrogen sulphide is 17.1 times
as heavy as a molecule of hydrogen. But a molecule of
hydrogen is believed to consist of two atoms ; hence a
molecule of hydrogen sulphide is 34.2 times as heavy as an
atom of hydrogen. Now, if this gas consists of one atom
of hydrogen in combination with one atom of sulphur, then
the atomic weight of sulphur will be the same as its equiva-
lent, viz., - ^=16; but if it contain two atoms of
ij.oo

hydrogen, then the atomic weight of sulphur will be

x-~— -- =32. The first hypothesis is impossible, for then

the molecular weight of hydrogen sulphide would be

ISOMORPHISM 17

16+ i = 17 ; whereas, it has been found to equal 34.2 ;
but if two atoms of hydrogen are present in sulphide of
hydrogen, the molecular weight is 32 + 2 = 34 ; and this is
nearly the same as the number found, viz., 34.2. It is,
however, still possible that hydrogen sulphide consists of two
atoms of hydrogen in union with two atoms of sulphur, in
which case the atomic weight of sulphur might still be 16 ;
but many other gaseous or gasifiable compounds of sulphur
are known, and in none of them is the molecular weight
such that they could be supposed to contain less than 32
parts of sulphur for each part of hydrogen, or its equivalent
of another element. It is consequently regarded as impro-
bable that the atomic weight of sulphur is less than 32 ; and
that 32 is the correct number follows also from the deter-
mination of its specific heat.

Isomorphism. — A third method of arriving at the
correct atomic weight of an element was suggested in 1819
by Billiard Mitscherlicll, then Professor in Berlin. When
two substances crystallise in the same crystalline form,
they are said to be «« isomorphous " with each other. It is
often the case that such compounds are similar chemically ;
that is, they may contain the same number of atoms, and
may also closely resemble each other physically. Thus,
there is a large class of compounds, named " alums," which
are sulphates of two metals. Ordinary alum is a sulphate
of aluminium and potassium ; it crystallises in eight-sided
regular figures, termed "octahedra." When the rare metal
gallium was discovered, it was found to form an "alum ; "
it gave a sulphate of gallium and potassium, crystallising in
octahedra, and similar in properties to ordinary alum. Now,
Mitscherlich's statement was, that when one element takes
the place of another in an isomorphous crystal of the same
chemical character, the substitution occurs so that one atom
of the one replaces one atom of the other, Hence, if the
atomic weight of the one element is known, the weight of
the other element which replaces it will be proportional to
its atomic weight. In the case above mentioned, it was

i8 MODERN CHEMISTRY

found that 27.1 parts by weight of aluminium were replaced
by 69.9 parts of gallium ; and as it was known from experi-
ments such as those previously described that the atomic
weight of aluminium is 27.1, it follows that 69.9 is the
atomic weight of gallium. But care is necessary in using
this indication of the atomic weight ; for it may happen that
two compounds may contain the same number of elements in
the same proportions, and have a similar crystalline form ;
and yet Mitscherlich's law may not be applicable.

CHAPTER II

Gaseous and Osmotic Pressure — Boyle's, Gay"
Lussac's, Pfeffer's, and Raoult's Laws.

IF we grant, in accordance with modern views, that matter
consists of minute particles, termed molecules, it must also
be allowed that the distance between these ultimate particles
must be very different, according to whether the matter is
in the solid, or liquid, or in the gaseous state. Thus, a
cubic centimeter of water at 100° expands, when it is
boiled into steam of the same temperature, to 1700 cubic
centimeters; and a cubic centimeter of oxygen, measured
at its boiling-point, -182°, boils into 266 cubic centimeters
of oxygen gas of the same temperature. In changing its
state, therefore, from liquid or solid to gas, matter under-
goes a great alteration of volume. It is accordingly to be
expected that the molecules of a gas, being at so much
greater a distance from each other than the molecules of a
solid or liquid, should yield more readily to pressure, and
should decrease in volume when the pressure is raised,
much more than solids or liquids. It is also found, as
appeared probable, that the expansion of a gas is much
greater than that of a solid or a liquid, by a definite rise
of temperature.

Boyle's Law. — The law relating to the compres-
sibility of gases was discovered by Boyle. It is, that if
temperature be kept constant, the volume of all gases
is inversely as the pressure. Thus, if the pressure of the
atmosphere, which is equal to 1033 grams on each square
19

20 MODERN CHEMISTRY

centimeter of the earth's surface at sea-level, or approxi-
mately i 5 Ibs. on each square inch, be doubled, the volume
of a given weight of air, or of any other gas, will be halved ;
on trebling the pressure the volume is reduced to one-third,
and so on. As the length of a column of mercury, one
square centimeter in cross-section, must be 76 centimeters
in order that its weight shall be 1033 grams, 76 centi-
meters is taken as the " normal " height of the barometer.
And if the height of the mercury in a gauge or " mano-
meter" is 152 centimeters, the pressure which produces
that rise in the mercurial column will halve the volume
of a gas exposed to it.

Gay*Lussac's Law. — The law connecting the
volume of a gas with the temperature was discovered by
Gay-Lussac, and independently by Dalton; but it is gene-
rally attributed to the former chemist. It is : — Provided
pressure be kept constant, the volume of a gas, mea-
sured at 0° C., increases by ^4^, f°r eac^- rise °f 1°-
Or i volume of gas at o° will become 1.00367 volume at
i°; 1.0367 volume at 10° ; 1.367 volume at 100°, and
so on. Generally stated, if / stand for a temperature, i
volume of gas will become i -f O.OO367/ when heated from
o° to that temperature.

A third law may be deduced from these two ; it is,
that if the volume of a gas be kept constant, the
pressure of a gas will increase ¥1^ of its initial
value at 0° for each rise of 1°. This is evident from
the following consideration : — Suppose that i volume of
a gas is heated from o° to i°; the volume will increase
to 1.00367 volume. To reduce the volume again to its
initial value, i, the pressure must be raised by 0.00367
of its original amount. If the initial pressure corresponded
to that of 76 centimeters of mercury, it would have to
be increased to 76 + (76 x 0.00367) centimeters, or to
76.279 centimeters in order that the gas should resume
its original volume of I. The same consideration will
hold if the gas is cooled instead of being heated ; but

DIFFUSION 21

of course in that case the pressure will be reduced, in-
stead of being raised. It follows from this, that if the
temperature could be reduced to 273 below o° C., the
gas would exert no pressure. This temperature, — 273°,
is termed " absolute zero." As a matter of fact, so low
a temperature has never been reached ; and, moreover, it is
certain that all gases would change to liquids before that
temperature was attained. But it serves as the starting-
point for what is termed the "absolute scale of tempera-
ture." Gay-Lussac's law may therefore be stated thus: —
The volume of a gas at constant pressure increases
as the absolute temperature ; and its corollary, thus : —
The pressure of a gas at constant volume increases
as the absolute temperature. For o° C. corresponds
with 273° on the absolute scale; and 273 volumes of gas
will become 274, if the temperature is raised from 273°
absolute to 274° absolute. Similarly, the pressure of a gas
will increase in the proportion 273 : 274 if the absolute
temperature is increased from 273° to 274°.

Pressure Proved by Diffusion. — When a solid
is dissolved in a liquid, as, for example, sugar in water, the
particles of sugar — 'its molecules — must obviously be
separated from each other to a greater or less extent, ac-
cording as much or little water be added. And it. has
been noticed that if the sugar be placed in the water, and
not stirred up, the sugar will dissolve at the bottom of the
vessel, and the strong solution of sugar will slowly mix up
with the upper layer of water, and in course of time be
equally distributed through the water ; just as a heavy gas,
such as carbonic acid gas, if placed in an open jar, will
gradually escape into the lighter air above it. This pro-
cess of mixing of two liquids or gases is termed "diffusion."
It is now generally held that the pressure of a gas on the
walls of the vessel which contains it is produced by the
impacts of its molecules against the walls ; and as the
molecules are extremely numerous, and in a state of very
rapid motion, they escape from an open vessel ; so that

22 MODERN CHEMISTRY

even a heavy gas, like carbon dioxide, will escape upwards
into a lighter gas ; and similarly, a light gas, like hydro-
gen, will escape downwards into a heavy gas, owing to the
unceasing motion of its molecules. The fact that the
molecules of sugar, which, by the way, becomes itself a
liquid when dissolved in water, travel upwards, and diffuse
through the lighter water, shows that they too are in
motion ; but the slowness of the diffusion, compared with
the rate of diffusion of a gas, indicates that their motion
is much impeded by the molecules of water, with which
they are constantly coming into collision. And just as the
motion of the molecules of a gas produces pressure, and
causes the gas to escape through an opening, so the motion
of the molecules of sugar, which causes them to rise through
water against the attraction of the earth, may be taken to
imply that they also exert a kind of pressure. But the
molecules of water, with which the molecules of sugar are
mixing, must also be held to exert pressure of the same
kind, since they disperse themselves through the molecules
of sugar. How is it possible to distinguish the pressure due
to the sugar from that due to the water ? A parallel case
with gases will help us to reply to this question.

Dalton's Law of Partial Pressures. — Suppose a
vessel of one litre capacity to be filled with oxygen gas at
o°, and under the atmospheric pressure of 76 centimeters of
mercury. The oxygen will exert pressure on its walls
equal to that of the atmosphere, for the vessel may be
placed in communication with the atmosphere, in order to
equalise pressure, before it is closed. Now let half a litre
of hydrogen be introduced by means of a force-pump. As
temperature and volume remain the same, the pressure will
be increased to 76 cms. + 38 cms. Introduce another half-
litre of hydrogen, and the initial pressure will be doubled; it
will now be 152 cms. Let another litre of hydrogen be
introduced, and the initial pressure will be trebled. We
might introduce a third gas, say nitrogen, into the vessel, and
the pressure would be increased proportionately to the quan-

LAW OF PARTIAL PRESSURES 23

tity introduced. Each constituent of the gaseous mixture,
accordingly, exerts pressure on the walls of the containing
vessel proportionally to its relative amount. For example,
the pressure of the nitrogen of the oxygen and of the argon
in air is proportional in each case to the amounts of these
constituents, viz., oxygen, about 21 per cent. ; nitrogen, 78
per cent. ; and argon, i per cent. This statement is known
as Dalton's law of partial pressures. If the pressure of
the air is 76 cms., that of the nitrogen is y7^ x 76 ; of the
oxygen, — -- x 76 ; and of the argon, —^ x 76 cms. In the
case of liquids, however, such a method fails. For while
in some instances the volume of a solution is nearly equal to
that of the solvent, plus that of the dissolved substance, in
others the volume is less, and in a few instances greater.

A device has, however, been discovered, by which it is
possible to measure the partial pressure of the dissolved sub-
stance ; and again an example will first be given from the
behaviour of gases. The rare metal palladium is permeable
at high temperatures by hydrogen, but not by other gases.
Now, if a vessel made of palladium be filled with a gas that
cannot escape through its walls — for example, with nitrogen
at atmospheric pressure — equal to that of 76 cms. of mercury
— and at a high temperature, say 300° C. ; and if it be
then surrounded with hydrogen gas, also at atmospheric
pressure, the pressure of the gases in the interior of the
vessel will rise to two atmospheres, owing to the entry of
hydrogen through the walls, which are permeable to that gas
alone. The mercury in the gauge connected with the
palladium vessel will rise, until it stands at a height of 76 cms.,
showing that the original atmospheric pressure has been
doubled. As there is no opposition to the passage of the
hydrogen inwards or outwards through the walls of the
vessel, hydrogen will enter until the pressure of the hydrogen
in the interior is equal to that on the exterior of the vessel.
But the nitrogen cannot escape, hence it exerts its original
pressure of 76 cms. of mercury.

Osmotic Pressure. — The partial pressure of the dis-

24 MODERN CHEMISTRY

solved substance in a solution has been measured by a similar
plan, devised by the German botanist Pfeffer. It was
necessary for this purpose to discover a " semi-permeable
membrane," through the pores of which water could pass
freely, but which would be impermeable to the dissolved
substance. A slimy precipitate, produced by adding
potassium ferrocyanide to copper sulphate, is not permeated
by dissolved sugar, though water freely penetrates it. But a
diaphragm of this nature is far too tender to withstand any
pressure. Pfeffer succeeded in depositing the slimy ferro-
cyanide of copper in the interior of the walls of a pot of
porous unglazed earthenware, and so constructing a vessel
which could be closed with a glass stopper, with the help of
cement. The stopper, which was hollow, was placed in
connection with a gauge containing mercury ; and after the
pot and stopper had been filled with a solution of sugar, the
stopper was connected with the gauge, which thus registered
the pressure upon, and consequently exerted by, the liquid.
The pot was then immersed in a large vessel of water, which
could be heated to any desired temperature, not too high to
soften the cement. It was found that the water slowly
entered the pot, and consequently raised the mercury in the
gauge ; but after a certain quantity had entered, the ingress of
water stopped, and the pressure ceased to rise.

The pressure thus raised has been termed " osmotic
pressure." The numbers which follow were obtained by
PfefFer :—

Concentration. Pressure. Ratio.

1 percent. 53.5 cms. 53.5

2 „ „ 101.6 „ 50.8

4 99 99 2°8'2 99 52'1

6 „ „ 3°7-5 99 5l-$

When a gas occupying a certain volume is increased in
quantity by pumping in an equal volume of gas, it is clear
that the number of molecules in the volume is doubled ;
and experiment shows that, in accordance with Boyle's law,

OSMOTIC PRESSURE 25

the pressure is doubled. The concentration of a solution
is expressed by the weight of dissolved substance in 100
parts of the solution ; and it is evident from PfefFer's
numbers that, on doubling the number of molecules of
sugar in a given volume of the solution, the osmotic
pressure is also doubled. The osmotic pressure, in fact,
increases directly as the concentration, exactly as with
gases.

PfefTer also made experiments at different temperatures.
Owing to the softening of the cement with which the semi-
permeable pot was closed, he was not able to use high
temperatures ; but some of his results are given below : —

Temperature Temperature
C. Abs.

Pressure.

Pressure
Calculated.

J5-50

• 273 = 287.2°
„ = 288.5°

51.0 cms.
52.1 „

51.0 cms.
51.2 „

32.0°
36.0°

„ - 3°5-°°
„ = 309.0°

54-4 »
56-7 „

54-1 »

54-9 99

The results are meagre, but, so far as they go, in reasonably
good accord. Experiments of this kind have seldom been
made, owing to the difficulty in preparing satisfactory
membranes. The calculation has been made on the assump-
tion that the osmotic pressure, like the gaseous pressure,
increases directly as the absolute temperature.

A striking proof of the correctness of the analogy
between osmotic and gaseous pressure is derived from the
following consideration : A gram of oxygen gas, measured
at o° C. and 76 cms. pressure, has been found to occupy
699.4 cc. ; now, 32 grams of oxygen form a gram-
molecule, for the atomic weight of oxygen is 16, and
there are two atoms of oxygen in a molecule of the gas,
as we have seen on p. 13. The volume of 32 grams is
accordingly 699.4x32 = 22,380 cc. The simplest for-
mula for cane-sugar is C12H22On, and as the atomic
weight of carbon is 12, the molecular v/eight of sugar is
at least (12 x 12) + (22 x i) + ( 1 1 x 16) = 342. If it

26 MODERN CHEMISTRY

were possible for cane-sugar to exist in the state of gas, it
might be expected that 342 grams in 22,380 cc. would
exert the same pressure as 32 grams of oxygen, viz.,
76 cms., since 342 grams of sugar are likely to contain
as many molecules as 32 grams of oxygen. But sugar
chars when heated, and decomposes. However, it is
possible to calculate, by means of Boyle's and Gay-
Lussac's laws, the pressure which a i1 per cent, solution
of sugar ought to exert at 14.2° C. If there were
223.8 grams in 22,380 cc., the solution would be one
of I per cent. And the pressure which it should exert

22 3 8

would be x 76, or 51.66 cms. at o° C., or 273°

Abs. And at 14.2° C., or 287.2° Abs., this pressure
should be increased in the proportion 273 : 287.2 ; giving
a theoretical pressure of 52.5 cms.; the actual pressure
measured was 51 cms. — a fairly close approximation. It
may, therefore, be taken that sugar in solution in water
exerts the same osmotic pressure on the walls of a semi-
permeable vessel, as the same number of molecules would
do, if it were in the state of gas, occupying the same
volume, and at the same temperature.

Experiments with semi-permeable diaphragms arc very
difficult ; the diaphragm seldom receives sufficient support
from the pipe-clay walls of the pot, and is usually torn
when the pressure rises to even a very moderate degree.
But it is not necessary to attempt such measurements ; for
the Dutch chemist, J. H. van't Hoff, now Professor of
Physical Chemistry in Berlin, pointed out in 1887 that
very simple relations exist between the osmotic pressure of
solutions and the lowering of the freezing-point of the
solvent, due to the presence of the dissolved substance,
and also the rise of boiling-point of the solvent, produced
by the same cause. A proof of this connection will not be
attempted here, but the facts may be shortly stated.

Measurement of Osmotic Pressure by Lower*
ins of Freezing=point. — All pure substances have a

DEPRESSION OF FREEZING-POINT 27

perfectly definite melting-point ; thus, ice melts at o° C.,
sulphur at 120°, tin at 226°, lead at 325°, and so on.
These temperatures are also the freezing-points of the
liquids, provided some of the solid substance is present.
If this is not the case, then it is possible to cool the liquid
below its freezing-point without its turning solid. Accord-
ingly, water freezes at o° if there is a trace of ice present ;
melted tin solidifies at 226° if there is a trace of solid tin
added to the cooled liquid ; and if, for example, water be
cooled without the presence of ice, until it has a tempera-
ture lower than o°, say 0.5° below o°, on addition of a
spicule of ice a number of little crystals of ice begin to
form in the liquid and the temperature rises to o°. But
if there is some substance dissolved in the liquid, as, for
example, sugar in the water or lead in the tin, then the
freezing-point is lowered below that of the pure substance.
And when the solvent freezes, in general the solid consists
of the solid solvent, none of the dissolved substance crystal-
lising out with it. It is owing to this fact that travellers in
Arctic regions manage to get water to drink ; for the ice
from salt water is fresh, and when melted yields fresh water.
It has been observed that with the same solvent the
freezing-point is lowered proportionally to the amount of
dissolved substance present, provided the solution is a
dilute one. Thus, a solution of cane-sugar in water,
containing 3.42 grams of sugar in 100 grams of the
solution, froze at 0.185° below zero; and one contain-
ing half that quantity, 1.71 grams, froze at 0.092° below
zero. Again, the same lowering of the freezing-point is
produced by quantities proportional to the molecular
weights of the dissolved substances. Malic acid, an acid
contained in sour apples, has the molecular weight 134,
while it will be remembered that the molecular weight of
cane-sugar is 342. Now, a solution of 1.34 grams of
malic acid in water, made up with water so that the whole
solution weighed 100 grams, froze at 0.187° below zero, a
number almost identical with that found for sugar.

28 MODERN CHEMISTRY

Solvents other than water may also be used ; but in
that case the lowering of the freezing-point is different.
Acetic acid, which is vinegar free from water, is often
employed ; so also is benzene, a compound separated from
coal-tar, produced in the manufacture of coal-gas. The
freezing-point of acetic acid is 17°; that of benzene is
4.9°. It was found in 1884 by Raoult, Professor of
Chemistry in Grenoble in the South of France, that while
1.52 grams of camphor (the hundredth part of its mole-
cular weight) dissolved in benzene (100 grams of solution)
lowered the freezing-point of the benzene by 0.514°, the
same quantity of camphor, forming a solution in acetic acid
of the same strength, lowered the freezing-point of the
latter by 0.39°. And he also noticed that the lowering of
the freezing-point is proportional, at least in some cases, to
the molecular weights of the solvents. Thus, the mole-
cular weights of acetic acid and benzene are respectively
60 and 78; and as 0.39 : 0.514 : : 60 : 79, the pro-
portionality is very nearly exact.

It is possible by this means to determine the molecular
weight of any substance which will dissolve in any solvent
for which the depression produced in the freezing-point is
known. Thus, for example, Beckmann, the deviser of
the apparatus with which such determinations are made,
found that a solution of naphthalene, a white compound of
carbon and hydrogen contained in coal-tar, in benzene, the
solution containing 0.452 per cent, of naphthalene, lowered
the freezing-point of benzene by 0.140°. A I per cent,
solution would therefore cause a lowering of 0.309°. And
as 0.309 : 0.39 :: 100 : 126, this is therefore the molecular
weight of naphthalene. The simplest formula for naphtha-
lene is C5H4, for its percentage composition is carbon,
93.75, hydrogen, 6.25; and to find the relative number
of atoms, the percentage of carbon must be divided by the
atomic weight of carbon, and that of hydrogen by its atomic

weight, thus : — 25^75 = 7.81, and _— 5 = 6.25 ; and these

RISE OF BOILING-POINT 29

numbers are to each other in the proportion 5 : 4. But
a substance with the formula C5H4 must have the molecular
weight (5x 12) + (4 x i)=64; whereas the molecular
weight found is 126. Now, 126 is nearly twice 64;
hence the formula of naphthalene must be C10Hg. The
method is not exact, but it affords evidence which, taken
in conjunction with the analysis of the compound, enables
the molecular weight to be determined.

Measurement of Osmotic Pressure by Rise
of Boiling=point. — A method for determining the mole-
cular weights of substances by the rise of boiling-point of
their solutions was also devised by Beckmann, and it is
frequently used. The process is analogous to that in
which the depression of freezing-point is made use of.
Every pure substance has a perfectly definite boiling-point,
provided that pressure is constant ; but if any substance is
dissolved in a pure liquid, the boiling-point of the latter
is raised ; and it is found that the rise of boiling-point is
proportional to the number of molecules of the dissolved
substance present. As an example, let us calculate the
molecular weight of iodine dissolved in ether from the
rise in the boiling-point of the ether. The rise caused by
the hundredth part of the molecular weight of a substance
taken in grams, and dissolved in 100 grams of ether, is
0.2105°. Now, Beckmann found that 1.513 grams of iodine
dissolved in 100 grams of ether raised the boiling-point
of the ether by 0.126°. And to raise the boiling-point by
0.2105°, 2.53 grams of iodine would have been necessary;
2.53 is therefore the hundredth part of the molecular
weight of iodine. It is possible to weigh iodine in the
state of gas, for it is an easily volatilised element ; and its
vapour has been found to be 126 times as heavy as hydro-
gen. We have seen that this statement implies that a
molecule of iodine gas is 126 times as heavy as a molecule
of hydrogen gas ; and as a molecule of hydrogen consists
of two atoms, a molecule of iodine gas is 252 times as
heavy as an atom of hydrogen, or its molecular weight

30 MODERN CHEMISTRY

is 252. The number obtained from the density of the gas
is accordingly almost identical with that obtained from the
rise in the boiling-point of ether.

We have now studied four methods by means of which
the molecular weights of elements and compounds have
been ascertained ; they are : —

1 I ) By determining the density of the substance in the
state of gas with reference to hydrogen, and doubling the
number obtained ; for molecular weights are referred to the
weight of an atom of hydrogen, while a molecule, it is
believed, consists of two atoms.

(2) By measuring the osmotic pressure exerted by a
solution of the substance, and comparing the pressure with
that exerted by an equal number of molecules of hydrogen,
occupying the same volume, at the same temperature.

( 3 ) By comparing the depression in freezing-point of a
solvent containing the substance in solution, with the de-
pression produced by the hundredth part of the molecular
weight in grams of a substance of which the molecular
weight is known, and by then making use of the known
fact that equal numbers of molecules produce equal depres-
sion in the freezing-point of a solvent.

(4) By a similar method applied to the rise in boiling-
point of a solvent caused by the presence of a known
weight of the substance of which the molecular weight is
required.

CHAPTER III

Dissociation — Electrolytic Dissociation or
lonisation.

Dissociation. — A certain number of substances are
known which apparently do not conform to the laws which
have been explained in the last chapter. For example, the
compound of ammonia with hydrochloric acid, which has
the formula NH4C1, should have the density 26.75, f°r
the atomic weights of the elements it contains are N= 14 ;
H= i ; Cl= 35.5 ; and the molecular weight is the sum
of 14 + 4+ 35.5 = 53.5. But the found density is only
one quarter of this number, viz., 13.375. It was at first
imagined that this discrepancy was to be explained by
abnormal expansion of the gas ; but with such a supposition,
of course, Avogadro's law could not hold. Other sub-
stances which show the same " abnormal densities" are
pentachloride of phosphorus and sulphuric acid. To ex-
plain this abnormality, Henri Saint- Claire Deville pro-
pounded the idea that such substances do not go into the
state of gas as compounds, but that they split into simpler
components, each of which has its usual density, and a
mixture of the components will exhibit a mean density.
Thus, if ammonium chloride be imagined to decompose into
ammonia and hydrogen chloride on changing into gas, then
the density of the supposed ammonium chloride gas will be
the mean of the densities of its two constituents. Ammonia
has the formula NH3, and hydrogen chloride, HC1 ; the
former has the density 8.5, and the latter, 18.25 » an^ tne

32 MODERN CHEMISTRY

mean of these two numbers is 13.375. Phosphoric chloride,
which has the formula PC15, splits in a similar manner into
PC13 and C10 ; and sulphuric acid, H2SO4, into water,
H26, and sulphuric anhydride, SOg. To this kind of
decomposition, where the bodies which are decomposed by
a rise of temperature re-unite on cooling to form the origi*
nal substance, Deville gave the name dissociation. It has
been found possible, by taking advantage of the fact that
light gases, like ammonia, pass out through an opening,
or, as it is termed, " diffuse" more rapidly than heavier
gases, like hydrogen chloride, to separate these gases,
and thus to prove that they exist as such in the vapour
of ammonium chloride ; for compounds are not decom-
posed into their constituents by diffusion ; hydrogen chlo-
ride diffuses as such, and is not split into hydrogen and
chlorine.

Let us look at this dissociation from another standpoint.
We know that if 2 grams of hydrogen, or 32 grams of
oxygen, or 28 grams of nitrogen, or, in fact, the molecular
weight of any gas expressed in grams, be caused to occupy
22,380 cubic centimeters at o° C., the pressure exerted by
the gas will be 76 centimeters of mercury. If the tempe-
rature is higher, the pressure will be increased proportionally
to the increase in absolute temperature. Thus, suppose the
'temperature were 300° C., the pressure would be increased
in the proportion 273° Abs. : 573°Abs. :: 7 6 cms. : 1 60 cms.
Now, if 53.5 grams of ammonium chloride were placed in
a vacuous vessel of 22,380 cc. capacity, and the temperature
were raised to 300° C., and if no dissociation were to
take place, one would expect a pressure equal to that of
1 60 cms. of mercury. It has been found, however, that
the actual pressure is twice that amount, or 320 cms. In
order to account for the doubled pressure, the supposition
that dissociation has taken place must again be made ; that
is, in order that the pressure must be doubled, twice as many
molecules must be present as one would have supposed from
the weight taken. The fact of dissociation may accordingly

DISSOLVED SALTS 33

be inferred either from a diminished density or from an
increased pressure.

"Dissociation" of Salts in Solution. — Few

measurements of the osmotic pressure of salts have been
made, owing to the difficulty in producing a membrane which
shall allow water to pass, and which shall be impermeable
to salts. But very numerous measurements of the depression
in freezing-point and the rise in boiling-point of solutions of
salts have been made ; and it has been already explained
that these quantities are proportional to the osmotic pressure
of the dissolved substances. It has been experimentally
discovered that in all such cases the fall in freezing-point,
or the rise in boiling-point is too great for the supposed
molecular weight of the salt. It must be concluded that
the osmotic pressure would also be increased, were it possible
to measure it. But the fall in freezing-point or the rise in
boiling-point does not imply a doubled osmotic pressure,
when there is reason to expect it, unless the solution is very
dilute. Now, if the pressure were doubled, we might argue
from such cases as ammonium chloride that dissociation into
two portions had occurred ; but in moderately concentrated
solutions, as the pressure is not doubled, it must be concluded
that the dissociation is not complete ; it is only in very
dilute solutions that complete dissociation can be imagined to
have taken place. Cases are known where substances in the
state of gas undergo gradual dissociation, and then the pres-
sure does not attain its maximum until the temperature has
been sufficiently raised or the pressure sufficiently re-
duced. The reason that this is not noticed with ammonium
chloride is that the temperature of complete dissociation has
been reached before the substance turns to gas.

Common salt is chloride of sodium ; its formula is
NaCl ; and for long the suggestion that it dissociated
into an atom of sodium and an atom of chlorine on being
dissolved in water was received as too improbable to be
worth consideration. There is, of course, another way
out of the difficulty ; it is to suppose that a molecule of

VOL. i. C

34 MODERN CHEMISTRY

salt has the formula Na9Cl2 ; in that case, 117 grams of
salt — (2 x 23) + (2 x 35.5) — dissolved in 10,000 grams
of water should produce the normal lowering of freezing-
point ; or, if it produced a larger lowering, it might be
supposed that these complex molecules had split more or
less completely into the simpler molecules, NaCl. But
though the explanation suggested might account for this
instance, it is incapable of accounting for the fact that
chloride of barium, which is known to possess the formula
BaCl2 (or a multiple thereof), gives, in sufficiently dilute
solution, a depression three times that which one would
have expected from its supposed molecular weight, or that
ferricyanide of potassium and ferrocyanide of potassium,
the formulae of which are respectively K8Fe(CN)6 and
K4Fe(CN)6, should give four and five times the expected
depression. But these results are quite consistent with the
hypothesis that

NaCl + Aq decomposes into Na. Aq and Cl. Aq ;
BaCl2 + Aq decomposes into Ba. Aq and Cl. Aq, and Cl. Aq ;
K3Fe(CN)6 + Aq decomposes into K.Aq + K.Aq + K. Aq,

and Fe(CN)6.Aq; and
K4Fe(CN)6 + Aq decomposes into K.Aq + K.Aq + K.Aq

+ K.Aq, and Fe(CN)6.Aq.

(The symbol " Aq " stands for an indefinite but large
amount of water — "aqua.") Here again we are face to
face with facts and an attempted explanation. The facts are
that certain compounds, which have long been known as
" salts," give too great a depression of the freezing-point
or too great a rise of boiling-point of the solvent in which
they are dissolved, corresponding to too great an osmotic
pressure. It has been observed that when the dilution is
sufficient the depression in each case reaches a maximum,
and that that maximum is two, three, four, or five times
what might be expected ; and in each case it is possible to
divide the salt into two, three, four, or five imaginary por-
tions, which often consist of atoms, though frequently of
groups of atoms.

ELECTROLYSIS 35

Electrolytic Conductivity of Salt Solutions.—

This hypothesis, that a kind of dissociation takes place in
salt solution, might have failed to gain acceptance had it
not been for a very remarkable coincidence. It appears
that all solutions which show this behaviour allow an electric
current to pass through them, whereas all solutions of com-
pounds such as cane-sugar, do not permit the passage of a
current of electricity. The latter class of compounds is
called " non-conducting ; " the former, class contains com-
pounds which are " conductors " of electricity. But metals
and certain compounds, chiefly consisting of the sulphides
of the metals, are also conductors of electricity, with this
difference, however : while the latter are apparently un-
altered by the passage of the electric current, solutions of
salts undergo profound change. In some cases, oxygen
appears in bubbles at the plate connected with the positive
pole of the battery, while hydrogen is evolved from that
connected with the negative pole ; in others, when the dis-
solved substance is a salt of such metals as copper, silver, or
mercury, the metals themselves are deposited on the negative
pole, or, as it is usually termed, the " kathode ; " while if
chlorine, bromine, or iodine is one of the constituents of
the salt, it is evolved at the " anode " or positive pole.

Faraday's Law. — It was discovered in 1833 by
Michael Faraday, Professor of Chemistry in the Royal
Institution in London, that if an electric current be passed
simultaneously through different solutions, the weights of metals
deposited or of elements or groups of elements liberated are
proportional to their equivalents (see p. I 5 ). If the same cur-
rent be passed, for example, through a solution of dilute sul-
phuric acid, copper sulphate, and iodide of potassium, each
contained in its own vessel, provided with plates of platinum
or some other unattackable metal dipping into the solution,
for every gram of hydrogen evolved from the kathode in
the vessel containing sulphuric acid, 8 grams of oxygen are
evolved from the anode ; 32.7 grams of copper are de-
posited on the kathode dipping into the copper solution,

36 MODERN CHEMISTRY

while 8 grams of oxygen rise in bubbles from the anode ;
and lastly, 127 grams of iodine are liberated from the anode
in the vessel containing potassium iodide, I gram of hydrogen
rising from the kathode. The evolution of hydrogen
instead of the deposition of potassium is due to the fact
that the metal potassium is unable to exist in presence of
water, but immediately displaces its equivalent of hydrogen.
All these numbers are in the proportions of the equivalents
of the elements. And without the liberation of these
elements no current passes. The elements may, there-
fore, in a certain sense, be said to convey the electricity ;
and as the same quantity of electricity passes through each
solution, liberating equivalents of the elements in each case,
it would appear that the same quantity of electricity is con-
veyed by quantities of elements proportional to their equiva-
lents. The equivalent of an element, it will be remembered,
is the weight of the element which can combine with or re-
place one part by weight of hydrogen ; it may be identical
with, or it may be a fraction of the atomic weight. In
the instances given above, the equivalents of iodine and of
potassium are identical in numerical value with their atomic
weights ; but those of oxygen and of copper, 8 and 32.7,
are half their atomic weights, which are respectively 1 6 and
63.4. It would follow, therefore, that an atom of copper
or of oxygen is capable of conveying a quantity of electri-
city twice as great as that conveyed by an atom of hydrogen
or of iodine.

But how is it known that the atoms " convey " quantities
of electricity ? Must they be imagined as like boats, taking
in their load of electricity at one pole, and ferrying it over
to the other, and there discharging ? It was at one time
held that the process rather resembled the method of
loading a barge with bricks, where a row of men, who
may stand for the atoms, pass bricks, representing the
electricity, from one to the other. But it was proved
by Hittorf that the charged atoms actually travel or
"migrate" from one pole to the other, carrying with

TRANSPORT OF ELECTRICITY 37

them their electric charges. And the charged atoms, for
which the name " ions," or "things which go," was de-
vised by Faraday, do not always move at the same rates.
The rate of motion depends on the friction which the ions
undergo on moving through the water or other solvent in
which the salt is dissolved. This friction is different for
different ions ; it also depends on the particular solvent em-
ployed ; and it is diminished if the temperature is raised.
The force which impels the ions is the same as that commonly
known as electric attraction and repulsion ; the negatively
charged atoms or "kations" being repelled from the negative
and attracted by the positive electrode dipping into the solu-
tion, while the positively charged atoms or "anions" are
repelled by the anode and attracted by the kathode.

When the anions touch the kathode, they are discharged ;
and similarly, when the kations touch the anode, they lose
their charge. And for every anion discharged, a kation
must simultaneously lose its charge. The result of this
is that the number of anions remaining in solution must
always be equivalent to the number of kations. It need
not always be the same, for it is possible for a kation
like copper to carry twice the charge of an anion like
chlorine ; but the number of " electrons," or electric
charges, must always be the same, although some ions
are capable of carrying more than one electron. There
can never, therefore, be an excess of, say, copper ions
in solution ; for they are always balanced by the requisite
number of anions. Thus, if the solution be evaporated,
the remaining salt has its usual composition ; though, of
course, there is less of it than if none had been decomposed.

Hittorf s Migration Constants. — The fact that ions
move at different rates can be demonstrated in two ways,
one direct, the other indirect. The indirect method was
devised by Hittorf; the direct method, which is much
more recent, was first suggested by Lodge.

It is always advisable to form a mental picture, if
possible, of any physical phenomenon, pour preciser les

38 MODERN CHEMISTRY

idee*) as the French say ; and a trivial illustration will be
now given which may render HittorFs conception clearer.
Imagine a ball-room with a door at each end. Suppose
the partners to be all separated from each other ; and
suppose an order to be given that the men shall march to
one door at twice the rate at which the ladies make for
the other door ; but that at the same time, for every man
who passes through the one door, only one lady shall
pass through the other door. At a given signal, say when
half the ballroom has escaped, let the condition of the
room be examined. It is easy to see that there will be
an equal number of men and women in the room, but
that there will be a greater number round the door at
which the men issue than round that at which the ladies
are trying to escape. And the rates of motion will be
proportional to the relative numbers in each half of the
ballroom, for the greater the rates at which the men move
proportionally to the ladies, the greater will be the number
in that part of the room at which the men are escaping.

This is a conception in close analogy with Hittorf's.
The men and women are the anions and kations ; and on
analysing the solutions round the anode and kathode, he
found that the concentration was, as a rule, altered, so
that he was forced to conclude that the rate of motion
towards the pole at which the concentration was increased
was more rapid than that towards the pole at which he
found the concentration to be diminished. By comparing
the concentrations, too, he calculated the relative rates of
motion of the anions and the kations towards the kathode
and the anode respectively.

Lodge's direct method has recently been improved by
Orme Masson, and very accurate results have been ob-
tained by him. His plan is to trace the rate of motion
of the anions by following them up with a coloured anion,
such as the copper ion, which is blue, and can be seen,
while the rate of motion of the kation is indicated by
following it up with a coloured kation : the one he used

MIGRATION OF IONS 39

for this purpose is the chromate ion, which is orange-yellow.
The apparatus which Masson employed consisted of two
flasks connected together by a narrow tube. This tube
is rilled with a solution of the salt of which the rate
of migration of the ions is to be determined, but in order
to prevent diffusion of the liquid, or escape owing to
currents produced by differences of temperature, the water
in which the salt is dissolved contains enough gelatine to
make it set into a jelly when cold. It is found that the
gelatine does not appreciably interfere with the motion of
the ions. The one flask was charged with a solution of
copper chloride, and the anode plate was of copper. The
other flask was charged with a dilute solution of a mixture
of chromate and bichromate of potassium, and the kathode
was of platinum. The connecting tube was filled with a
warm solution of the salt to be examined, say potassium
chloride, in water containing gelatine, and after it had
cooled and set it was placed in position. On passing the
current, the potassions migrate towards the kathode, and
are followed closely by the blue cuprions, which serve to
mark the position of the rearmost of the potassions. The
chlorions, on the other hand, migrate towards the anode,
followed by the orange-yellow chromations, which reveal
their position. The rates can be measured by following
the advance of the colour in the tubes. If the ions have
equal velocity, as is nearly the case with potassions and
chlorions, the meeting-place of the blue and the orange is
nearly at the middle of the tube ; but if, as in most other
cases, the rates are different, the point of junction will be
at one side or other of the middle point of the tube. The
distances traversed in the same time give a direct measure
of the relative velocities of the anion and kation. Having
established this ratio, another salt, say sodium chloride,
having a different anion but the same kation, can be em-
ployed, and so the relative rates of potassion and sodion
may be compared.

The table which follows gives the rates of migration of

40 MODERN CHEMISTRY

a few ions compared with that of potassion, which is taken
as 100.

K Na Li NH4 Mg/2 Cl SO4/2
100 65.6 45*0 100 40.5 97-O 87.7

As the conductivity for a current depends on the velocity
both of the anion and the kation, relative numbers for the
conductivity may be obtained for any salt by adding the
numbers of the individual ions given above. Thus, if it is
required to find the conductivity of lithium sulphate, which

SO

has the formula Li2SO4, we have Li = 45, and ^=Sj.fjt

together equal to 132.7.

Measurement of the Extent of lonlsation. — It

is found in practice, however, that the conductivity of salts
agrees with the numbers deduced from the velocity of their
ions only when the solution is a very dilute one, and even
then not always. This can be ascribed to either or to both
of two causes. If the solution is a strong one, the mole-
cules of salt may bear an appreciable ratio to the molecules
of water, and may interfere by their possibly greater fric-
tion with the free transit of the ions. Or, on the other
hand, some of the molecules may not be resolved into ions,
and there may be fewer " boats " to carry across the elec-
trons, the progress of which towards the anode and the
kathode must consequently be slower, for the non-ionised
molecules take no share in the conveyance of electricity.
And as the conveyance of electricity depends on the number
of ions and on the rate at which they move, if the latter is
known, the relative number of ions may be calculated from
measurements of the conductivity of the solution.

Conductivity of Electrolytes. — To measure the
conductivity of a solution, a "gram-molecular-weight,"
i.e. the molecular weight of the salt taken in grams, is
dissolved in a litre of water. A small quantity of this
solution is placed in a small beaker immersed in a large
tank of water, so that the temperature may not vary ; for

ELECTRIC CONDUCTIVITY 41

the conductivity increases with rise of temperature, owing
to the smaller resistance offered by hot water to the passage
of the ions, than by cold. Two circular platinum plates,
the surfaces of which have been roughened by having
platinum deposited on them, are immersed in the solution,
so that one lies at the bottom of the beaker, while the other
is one centimeter distant from it, higher up in the liquid.
The wire connected with the lower plate is protected by a
glass tube, in order that the current may pass only between
the plates. To measure the conductivity of this solution an
arrangement termed a " Wheatstone's bridge " is employed,

the construction of which will be understood from the an-
nexed diagram. B is a battery, actuating a small toy coil, C,
from the secondary terminals of which T T wires proceed
to the measuring-bridge Mb, which consists of a straight
piece of nickel-silver wire stretched along a scale. From
one end of the bridge, b, the current traverses the solution ;
from the other, M, the current passes through a resistance-
box, R, containing bobbins of wire of known resistance, the
number and resistance of which can be varied at will until
the resistance is nearly equal to that of the solution. If
they are exactly equal, and if the pointer P is exactly in

42 MODERN CHEMISTRY

the middle of the bridge-wire Mb, then no sound can be
heard in the telephone T. The resistance of the solution
can then be read from the box. If, as generally happens,
the resistance of the solution is not equal to that in the box,
it is necessary to move the pointer P until the resistances are
equal. Having thus ascertained the resistance of the solu-
tion, a portion is diluted with water, so that its strength is
exactly half of the former, and the resistance is again deter-
mined. Successive dilutions in which the volume of the
solution is doubled, and again doubled, are made, and in
this manner the resistance due to an equal number of mole-
cules in each case is calculated. The conductivity is the
reciprocal of the resistance, and it is found that the mole-
cular conductivity increases with the dilution up to a certain
point. Thus Kohlrausch, the deviser of this method, found
the following numbers for sodium chloride : —

Concentration : Molecular Relative number of ions

58.5 grams in conductivity, per TOO molecules of salt.

1 litre 69.5 67.5

2 litres 75.7 73.6
10 „ 86.5 84.1

100 „ 96.2 93.5

1,000 „ 100.8 98.0

10,000 ,, 102.9 100.0

50,000 „ 102.8 100.0

It is evident that 58.5 grams of salt dissolved in 10,000
litres of water give a maximum conductivity, for the dilu-
tion to 50,000 litres is attended by no further increase.
That the altered velocity is not influenced by the frictional
resistance of the water is obvious, for the solution of 58.5
grams of salt in 10 litres of water does not differ much
from pure water in this respect. The increase in conducti-
vity must accordingly be attributed to an increase in the
number of ions at the expense of the molecules ; and, as
a dilution in 50,000 litres of water produces no greater
conductivity than in 10,000, it must be concluded that

CONDUCTIVITY OF WATER 43

complete ionisation has taken place. The figures in the
last column are obtained by simple proportion, thus : — As
102.9 : 69.5 :: I0° : 67.5.

The extent of ionisation calculated from the conducti-
vities of salt solutions agrees well in the main with that
calculated from the depressions of freezing-point.

Conductivity of Pure Water. — Two points remain
to be mentioned. One has reference to the conductivity of
pure water. It is no easy matter to prepare pure water ;
even after the water has been distilled, it contains traces
of substances which are ionised, such as carbonic acid or
ammonia. It is possible, by employing special precautions,
to add to the water before distillation substances which form
non-volatile compounds with these impurities, and by making
use of vessels of gold or platinum, which are not attacked,
however slightly, by water, to produce almost pure water.
Such water is not wholly devoid of conductivity, but its
resistance to the passage of electricity is very great. It
must be presumed that the water is for the most part mole-
cules of H2O, or perhaps even more complex molecules,
such as H4O2, H6O3, &c. But there are, besides, a few

+

ions of H and OH ; so that water is capable of reacting in
certain cases where ions might be suspected.

Conductivity of Pused Salts. — Another fact which
is well known, and largely put to practical use, is that fused
salts are, as a rule, good electrolytic conductors of electri-
city. Even when the salt is as pure as it can be made, it
still conducts in the molten state. Although the conducti-
vity of fused salts has not been investigated with the same
completeness as that of solutions, yet it cannot be doubted
that the salt must be more or less ionised. The ionisation
appears to increase with rise of temperature, for the salt
becomes a better conductor. This, however, may be due,
in part at least, to the smaller frictional resistance which it
offers to the passage of the ions towards the electrodes.
.But recent experiments have shown that the molecules of

44 MODERN CHEMISTRY

some salts, at least — those in which measurement is possible
— are more complex than would be supposed from their
formulae. Thus, nitre, KNO3, consists of molecules of
four times that complexity, or K4N4O12 ; and it is not
improbable that among these complex molecules there are

+

some ions of K and of NO3 capable of conveying an elec-
tric charge. It may indeed be stated that those liquids
which possess complex molecules have the power of ionis-
ing salts dissolved in them. Water is one of the most
striking examples ; and it is to be presumed that such com-
plex molecules are able to surround and prevent ions from
at once discharging into one another by protecting them
from each other.

To sum up : — Certain substances, in the state of gas,
exhibit dissociation — that is, they decompose into simpler
constituents, which combine again on cooling. This disso-
ciation is favoured by a rise of temperature or by a lowered
pressure, and reversed by a fall of temperature or a rise of
pressure. From a determination of the density of the mix-
ture of gases the extent of the dissociation can be calculated.
Certain substances, in like manner, and such substances are
generally named " salts," when dissolved in water or certain
other solvents, undergo electric dissociation or ionisation ;
this dissociation is often increased by a rise of temperature,
and always by dilution. The constituents of such solutions,
the anions and kations, can be urged in opposite directions
by an electric current ; they usually " migrate " at different
rates ; and when they discharge, by contact with the elec-
trodes, they are sometimes liberated in the free state, as, for
example, many metals, and bromine and iodine ; but some-
times the discharged ion is incapable of existing in the free
state in contact with the solvent, and in this case they
react with the water, and form new secondary compounds.
The amount of this ionisation can be measured by deter-
minations of the depression of freezing-point, or of the
conductivity, of the solution.

CHAPTER IV

Elements: — Methods of Preparation; Classifi-
cation ; Valency. Compounds : — Structural
Formulas; Classification; Nomenclature.

WE have seen, in Chapter L, how the idea of an " element "
as a constituent of compounds gradually became more de-
fined. As fresh discoveries were made, it was found that
certain substances could not further be decomposed, yielding
new constituents. But it is not easy always to determine
whether or no a substance is an element. For certain
compounds are very stable, that is, are very difficult to de-
compose ; and it has happened several times that such com-
pounds were mistaken for elements. A remarkable instance
is a copper-coloured body, found in the debris left in the
hearth of an old iron furnace, which was for long supposed
to be the element titanium ; more careful investigation,
however, proved it to be a compound of titanium with
nitrogen and carbon.

Methods of Preparing Elements. — There are
three methods by which elements have been prepared, and
all elements have been made by one of these methods.
They are : —

(i) Separation of the Element by Means of an
Electric Current. — We have already seen that the com-
pound must be ionised, and this is attained only by dissolv-
ing it in water or some other appropriate solvent, or by
fusing it. It is the act of solution or of fusion which
ionises the compound ; and the effect of the current is to

45

46 MODERN CHEMISTRY

direct the ions towards one or other electrode, and dis-
charge them ; they then assume the form of the free ele-
ment. It is necessary, in order that this method shall
succeed, that the discharged ion shall not act on the solvent,
nor on the electrode. It is impossible, for instance, to
deposit sodium from an aqueous solution of any of its salts,
for no sooner is the sodlon discharged than it is attacked by
the water ; hydrogen is evolved in equivalent amount to the
sodium, and sodium hydroxide is produced, in which the
sodium has taken the place of one of the hydrogen atoms in
water ; its formula is therefore NaOH. Chlorine, too,
cannot be produced by the electrolysis of a chloride, if the
anode is of iron, for example, for it at once unites with the
iron, and forms a chloride of that metal instead of coming
off as an element.

( 2 ) Separation of an Element from a Compound by
Heat. — There appears to be little doubt that at a suffi-
ciently high temperature all compounds would be decom-
posed into their elements : in the sun, which possesses a
temperature much higher than can be reached by any means
at our disposal, it is probable that all compounds are decom-
posed. But certain compounds, like silica or quartz, for
example, are so stable that they resist the highest tempera-
ture which we can give them, without any change, except
fusion and volatilisation. There is, moreover, another
reason why this process often fails to isolate an element.
The compound may be decomposed on heating, but its con-
stituents may re-unite on cooling, unless one of them is
more volatile than the other, and removes itself from the
sphere of action. For these reasons this process of obtain-
ing elements is of somewhat limited application. But it
forms the most convenient method of preparing oxygen ;
for example, if oxide of mercury be heated, it decomposes
into gaseous oxygen, the boiling-point of which lies far
below atmospheric temperature, —182°; while the mer-
cury, which boils at 358°, although it volatilises at the
temperature requisite to effect the decomposition of the

SEPARATION OF ELEMENTS 47

oxide, condenses in the flask or tube in which the oxide is
heated. Sulphide of gold, too, can be separated into gold and
sulphur on being heated; for while sulphur boils at 446°, the
boiling-point of gold is probably not much below 2000°.

(3) Separation of an Element from a Compound
by Displacement. — On heating one element with a com-
pound of another element, it not infrequently happens that
the element in combination is displaced and liberated, while
the other element takes its place in the compound. This is
doubtless an ionic phenomenon ; one element — that in com-
bination— being ionised, and hence electrically charged,
exchanges its charge with the added element, which in its
turn becomes ionised. A solution of iodide of sodium, for

+

example, contains todions and sections, I and Na. On
adding to it a solution of chlorine in water, in which there
are certainly many non-ionised chlorine molecules, C10, mole-
o-
cular iodine, I — I is set free, while ionised chlorine, Cl,
goes into solution. The free iodine forms a brown solu-
tion, or, if much is present, a black precipitate. Again,
when metallic sodium is heated with magnesium chloride to
a red heat, globules of metallic magnesium are set free,
while the sodium enters into combination with the chlorine.
It may be supposed that on fusion the magnesium chloride
contains some ions of chlorine and magnesium ; the non-
ionised sodium takes the charge of the ionised magnesium,
while the latter metal is liberated in an non-ionised state.
But it may be objected that only those magnesium ions
which exist as such should exchange their charges with the
sodium ; that is true ; but when they have done so others
become ionised and undergo a similar change ; for if the
temperature be kept constant, the ratio between the number
of the ionised atoms of magnesium and the non-ionised
atoms of magnesium in the chloride must remain constant,
so that when the magnesium ions are replaced by sodium
ions, other molecules of magnesium chloride become ionised
to keep up the balance.

48 MODERN CHEMISTRY

The element carbon is most frequently used to displace
other elements. In its case, little or nothing is known of
the electrical actions ; but if analogy may be taken as a
guide, its action may be attributed to a similar exchange
of electric charge between the displaced element and the
carbon. But here the carbon, as soon as it unites with the
oxygen which was previously in combination with the dis-
placed element, escapes in the form of gas, and the oxide of
carbon is certainly not an ionised compound.

An essential condition for the preparation of elements by
the method of displacement is that the element which it is
proposed to prepare in the free state shall not itself com-
bine with the element which is used to displace it. Thus,
chlorine cannot be used to displace either carbon or sulphur
from the compound of carbon with sulphur, bisulphide of
carbon, since it itself combines with both the carbon and
the sulphur, yielding chloride of sulphur together with
chloride of carbon. In general, however, this difficulty
does not occur.

The elements which are generally used for the displacement
of others from their compounds are : —

1. Free hydrogen at a red heat which displaces

elements from their oxides or chlorides.

2. Ions of hydrogen, on the point of being discharged

electrically, or hydrogen " in the nascent state," i.e.
hydrogen being set free from its compounds by the
action of a metal ; it also displaces elements frorn their
oxides or chlorides, or, in general, from their salts.

3. Metallic sodium, which displaces elements from their

chlorides or fluorides.

4. Metallic magnesium, which displaces elements from

their chlorides or oxides.

5. Metallic aluminium, which displaces elements from

their oxides.

6. Metallic iron, which displaces elements from their

sulphides.

7. Fluorine, which displaces oxygen from water ;

CLASSIFICATION 49

chlorine in sunlight, which acts slowly in the same
way ; chlorine displaces bromine, and bromine,
iodine.

8. Carbon, which is the most generally employed agent
for replacing other elements ; it combines with oxy-
gen, forming carbonic oxide or carbonic anhydride
gases, and liberating the element with which the
oxygen was combined.

The question of cost or of convenience often decides as
to which of these methods is used. In the sequel, only
the more generally used plan will be described. It must
be remembered, too, that the employment of these processes
does not always lead to the isolation of the element ; in
many cases a compound is produced, containing less of the
element which it was intended to remove ; and it is some-
times difBcult to decide whether or not an element has really
been set free. Experiments on its compounds are often
required to decide the question.

Classification of Elements. — For long it had been
noted that certain elements displayed a marked similarity
with each other. Thus the metals sodium and potassium,
discovered by Sir Humphry Davy, are both white, soft,
easily oxidisable metals, forming soluble salts with almost
all acids ; these salts resemble each other in colour, in
crystalline form, and in other properties. The subsequently
discovered metals, lithium, rubidium, and caesium, have also
a strong resemblance to potassium and sodium. Their
atomic weights also increase progressively ; thus we have
the series, Li = 7, Na= 23, K = 39-i, Rb = 85, and Cs =
133. Similar series had been noticed with calcium, stron-
tium, and barium ; magnesium, zinc, and cadmium ; and
so on. It was not until 1863 that John Newlands called
attention to the fact that if the elements be arranged in the
order of their atomic weights a curious fact becomes notice-
able. It is that, omitting hydrogen, the first, the eighth,
the fifteenth, and, in short, all elements may be so arranged
that the " difference between the number of the lowest
member of a group and that immediately above it is 7 ; in
VOL. i. D

50 MODERN CHEMISTRY*

other words, the eighth element starting from a given one is
a kind of repetition of the first, like the eighth note of an
octave in music." This idea was subsequently discovered
independently and elaborated by Lothar Meyer and by D.
Mendele'eff, and it has now been adopted, in spite of some
difficulties, as the ground-work of classification of chemical
substances.

The table may be given in the following form, although
there are many ways of representing the order in which the
elements lie : —

The Atomic Weights of the Elements arranged1
according to the Periodic System.

H

He

Li

Be

B

C

N

0

i

4

7

9

1 1

12

»4

16

F

Me

Na

Mg

Al

Si

P

S

19

20

23

24

27

28

31

32

Cl

A

K

Ca

Sc

Ti

V

Cr

Mn

35

40

39

40

44

48

5*

52

55

Br

Kr

Rb

Sr

Y

Zr

Nb

Mo

?

80

82

85

S7

89

90

94

96

98

I

X

Cs

Ba

La

Ce

Nd

Prd

Sm

127

128

i33

*37

I42

140

141

144

Yb

?

Ta

W

?

i73

182

184

Th

?

u

232

240

i

o

i

ii

iii

iv

v

v«

vii

to

to

to

to

vii

i

ii

ii

1 It is a matter of indifference which element is placed first on the
list. The most convenient form to give the diagram is that of two

PERIODIC TABLE 51

It will be noticed that the number of elements in the first
two horizontal rows is not seven, but eight, and that, con-
sequently, every ninth element, and not every eighth, pre-
sents similarity with its predecessor in the vertical columns.
This is owing to the recent discovery of the elements in
the second vertical column. It will also be seen that it ii>
possible, by folding the projecting slip to one side or other,
to bring new sets of elements in the third and succeeding
horizontal rows beneath the elements in the first and second.
The first and second rows are termed " short periods," the
others, " long periods." It appears that by so arranging
the elements, analogies are brought out more striking than
if such long and short periods were not adopted.

Valency. — The Roman numerals below the vertical
columns refer to what is termed the " valency." An
element capable of combining with or replacing one atom
of hydrogen, or, in other words, of which the equivalent
and atomic weight are identical (see p. 15), is termed a
monad, or is said to be monovalent. Thus, 23 grams of
sodium replaces I gram of hydrogen in water or in hydro-
gen chloride, to form hydroxide or chloride of sodium ;
and as 23 is known to be the atomic weight of sodium
from determinations of its specific heat, the atomic weight
of sodium is expressed by the same number as its equiva-
lent. It is therefore a monad. The element oxygen is a
dyad or divalent, because, in water, two grams of hydrogen
are combined with 1 6 grams of oxygen ; its equivalent is
therefore 8. But oxygen is 16 times as heavy as hydro-
gen— that is, a molecule of oxygen is 16 times as heavy as
a molecule of hydrogen ; and as a molecule of each of
these substances is believed to consist of two atoms, an
atom of oxygen is 16 times as heavy as an atom of
hydrogen. The atomic weight is therefore 1 6 ; but as

cylinders, on which the elements follow spiral lines, so that oxygen
and fluorine, sulphur and chlorine, follow each other round the
smaller cylinder, while selenium and bromine, tellurium and iodine,
&c. , are conspicuous round the larger cylinder.

50 MODERN CHEMISTRY*

other words, the eighth element starting from a given one is
a kind of repetition of the first, like the eighth note of an
octave in music." This idea was subsequently discovered
independently and elaborated by Lothar Meyer and by D.
Mendele'eff, and it has now been adopted, in spite of some
difficulties, as the ground-work of classification of chemical
substances.

The table may be given in the following form, although
there are many ways of representing the order in which the
elements lie : —

The Atomic Weights of the Elements arranged1
according to the Periodic System.

H

He

Li

Be

B

C

N

0

i

4

7

9

1 1

12

»4

16

F
*9

Me

20

Na
23

Mg
24

Al
27

Si

28

P

3i

S
32

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

Se

56

59

58.7

63

65

70

72

75

79

Rh

102

Ru
103

Pd
106

Ag

108

Cd

I 12

In
114

Sn
119

Sb

120

Te
127

?

?

?

P

?

Gd

?

?

?

156

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

?

191

X93

194

197

20O

204

207

208

vin vii iv in 11 iii iv v vi
to to to to and and and to to
ii ii ii i i i ii ii H

1 It is a matter of indifference which element is placed first on the
list. The most convenient form to give the diagram is that of two

PERIODIC TABLE 51

It will be noticed that the number of elements in the first
two horizontal rows is not seven, but eight, and that, con-
sequently, every ninth element, and not every eighth, pre-
sents similarity with its predecessor in the vertical columns.
This is owing to the recent discovery of the elements in
the second vertical column. It will also be seen that it is
possible, by folding the projecting slip to one side or other,
to bring new sets of elements in the third and succeeding
horizontal rows beneath the elements in the first and second.
The first and second rows are termed " short periods," the
others, " long periods/' It appears that by so arranging
the elements, analogies are brought out more striking than
if such long and short periods were not adopted.

Valency. — The Roman numerals below the vertical
columns refer to what is termed the "valency." An
element capable of combining with or replacing one atom
of hydrogen, or, in other words, of which the equivalent
and atomic weight are identical (see p. 15), is termed a
monad, or is said to be monovalent. Thus, 23 grams of
sodium replaces I gram of hydrogen in water or in hydro-
gen chloride, to form hydroxide or chloride of sodium ;
and as 23 is known to be the atomic weight of sodium
from determinations of its specific heat, the atomic weight
of sodium is expressed by the same number as its equiva-
lent. It is therefore a monad. The element oxygen is a
dyad or divalent, because, in water, two grams of hydrogen
are combined with 1 6 grams of oxygen ; its equivalent is
therefore 8. But oxygen is 16 times as heavy as hydro-
gen— that is, a molecule of oxygen is 16 times as heavy as
a molecule of hydrogen ; and as a molecule of each of
these substances is believed to consist of two atoms, an
atom of oxygen is 16 times as heavy as an atom of
hydrogen. The atomic weight is therefore 1 6 ; but as

cylinders, on which the elements follow spiral lines, so that oxygen
and fluorine, sulphur and chlorine, follow each other round the
smaller cylinder, while selenium and bromine, tellurium and iodine,
&c. , are conspicuous round the larger cylinder.

52 MODERN CHEMISTRY

the equivalent is 8, the atomic weight is twice the equiva-
lent. Hence the name " dyad" Similarly, there are tri-
valent elements, or triads ; tetravalent elements, or tetrads ;
penta*valent elements, or pentads ; hexavalent elements, or
hexads • heptavalent elements, or heptads ; and possibly one
octovalent element, or octad.

Valency of Elements. — As elements may have more
than one equivalent (see p. 15), so they may have more
than one valency. Certain elements, however, so far as is
known, possess only one valency ; examples of this are
found in the lithium, the beryllium, and the boron columns.
But the majority of elements exhibit more than one valency,
according to circumstances. Thus, compounds of nitrogen
are known possessing the formulae NO, NHg, NO2, and
NH4C1, in which one atom of nitrogen is combined with
one atom of dyad oxygen, and is therefore also a dyad ;
with three atoms of monad hydrogen, and is accordingly a
triad ; with two atoms of dyad oxygen, whence nitrogen is
here a tetrad ; and with four atoms of monad hydrogen
and one atom of monad chlorine — in all, with five monads
— and in this case nitrogen must be accounted a pentad.
The atomic weight of nitrogen is known from its density
to be 14; and its equivalents in these compounds are
respectively ~-9 ^, ^, and ^. This peculiarity makes
the classification of some of the elements a difficult task.

But there is an additional difficulty which meets us in
attempting to ascribe the valency to an element. It is
connected with what is known as the " structure " of
compounds. As this subject will be frequently alluded to
in succeeding chapters, enough will only be said here to
give an idea of the problem which faces us in attempting a
rational classification of the elements.

We are ignorant of the form of the atoms. It is true
that various speculations have been made which may
possibly lead to a true conception of their appearance and
motions, but these are not sufficiently definite and sup-
ported by facts to require more than a passing allusion

STRUCTURE 53

here. For all practical purposes, we are content, in
default of a better conception, to regard atoms as spheres,
(jfiarck and elastic, and compounds as formed by the
juxtaposition of these spheres. That this conception
is far from reality is more than probable, but it has
to suffice. Certain deductions, however, may be drawn
regarding the methods of combination of the atoms in the
molecule. It is certain that molecules must occupy space
of three dimensions ; but just as it is possible to represent
solid objects on a plane surface by the help of perspective,
so it is allowable to picture molecules as made up of atoms
spread over a plane surface, until we find facts which
demand space of three dimensions. We shall see later
that in certain cases such solid models of molecules are
necessary, but, as a rule, they can be dispensed with.
And instead of attempting to picture the atoms as circles
or projected spheres, the symbols alone will be employed.
The fact of combination will be indicated by a dash uniting
the atoms ; thus, a monad will have one, and only one,
dash, proceeding from it ; a dyad, two ; a triad, three, and
so on.

Structural Formulas. — The simplest case which
we can consider is that of a compound consisting of two
monovalent atoms, such as hydrogen chloride. Here we
have the structural formula, H — Cl. A compound of a
dyad with two monad atoms, such as water, or its analogue,
hydrogen sulphide, must have the formula, H — O — H,
or H — S — H. The compound of a triad with three
monad atoms, as, for example, ammonia, would be written

/H
H — N/ ; and of a tetrad with four monads,

where an atom of carbon is the tetrad, and the compound
is named methane, or "marsh-gas." The atom of sulphur
is, however, not always divalent ; it is sometimes tetra-
valent, as in its compounds with chlorine and with oxygen.
In the first case, tetra-chloride of sulphur has the formula,

54 MODERN CHEMISTRY

CL /CI

78^ ; and in the latter, sulphur dioxide is repre-
CK \C1

sented by the formula O = S = O. Sulphur dioxide unites
directly with chlorine on exposure of a mixture of the two
gases to sunlight, forming a compound named sulphuryl
chloride, which has the empirical formula, SO9C10 ; in
this compound sulphur is regarded as a hexad, hence the

ci o

structural formula must be /$C * Now, sulphuryl

CK ^O

chloride reacts at once with water when they are brought
into contact, and sulphuric acid is produced along with
hydrogen chloride. This change can be represented
structurally by the equation : —

H— O— H Ck ,O H-C1 H— Ov ,O

H— O— H Cl/ ^O H-C1 H— O/ V)'

The chlorine atoms of the sulphuryl chloride have com-
bined with two of the hydrogen atoms of two molecules of
water, leaving the residues — O — H, which are termed
" hydroxyl groups ; " these have taken the place of the
chlorine atoms, forming sulphuryl hydroxide, or, as it is
commonly termed, sulphuric acid. If the foregoing repre-
sentation is correct, then an intermediate substance should
exist, which may be named " sulphuryl hydroxy-chloride,"
and which should contain a chlorine atom and a hydroxyl
group, each in union with sulphuryl. Such a body has been
prepared by the direct union of sulphur trioxide, where
sulphur is in combination with three atoms of oxygen, with
hydrogen chloride. But here there must be a transposition
of the hydrogen atom, as is evident from the equation —

H ,,0 H-0V ^O

0 = S;<

CK

STRUCTURAL PROBLEMS 55

In a similar manner to the above schemes, the relations of
the atoms in compounds may be traced out, but sometimes
it is difficult to decide regarding the structure. Here is an
instance. The specific heat of the element barium shows
that it possesses an atomic weight not far removed from
137 ; the analysis of its chloride leads to the fact that
137/2 grams of barium are in combination with 35.5 grams
of chlorine, and 35.5 is known to be the equivalent of
chlorine; hence 63.5 is the equivalent of barium, and
63.5x2 = 137 is its atomic weight. Ordinary oxide of
barium corresponds with this, for it contains 137 grams of
barium in combination with 16 grams of oxygen ; hence we
accept barium as a dyad. But if barium oxide be heated to
dull redness in a current of oxygen, another atom of oxygen
combines with the oxide, and in the compound BaO0, 137
grams of barium are combined with 32 grams of oxygen.
Is barium a tetrad ?

Among all the numerous compounds of barium, no one is
known in which one atom of barium is combined with more
than two atoms of a monad ; when barium dioxide is
treated with hydrochloric acid, for example, two atoms of
oxygen are not replaced by four atoms of chlorine, but the
change is —

BaO2 + 4HC1 = BaCl2 4- H2O2.

Hydrogen dioxide is produced. Now the formula of
hydrogen dioxide has been proved by the freezing-point
method to be H2O2, and not HO ; hence it may be sup-
posed that it consists of two hydroxyl groups in union with
each other, thus : H — O — O — H ; in this case, barium

/°

dioxide would be Ba<^ j , the two atoms of oxygen being

themselves united together ; and there are many instances
of similar union. But it may also be held that one of
the atoms of oxygen is a tetrad, the other remaining a

56 MODERN CHEMISTRY

H\
dyad, thus : ;>O=O ; whence barium dioxide would be

H/

Ba=O = O. Both of these views can be supported by
arguments, and it is an open question which has a claim to
preference. It is certain, however, that barium is not a
tetrad.

In other instances, it must be confessed that the evidence is
by no means so clear, and there is then considerable doubt as
regards the correct classification of the elements concerned.

It must not be forgotten that we have as yet no clear
conception as to the cause of valency ; at present we accept
the facts, and endeavour to use them as a guide to the
classification of compounds.

Were all the elements to be capable of combining with
each other, it is easily seen that the number of compounds
would be prodigious, and that no mind could possibly hope
to grapple with them ; but it happens that only a certain
number of elements forms well-defined compounds with the
rest, and the grouping of compounds is thus not so diffi-
cult a task as might be supposed. The classes are the
following : —

Classification of Compounds : —

1. Hydrogen combines with a few elements, forming
hydrides.

2. Fluorine, chlorine, bromine, and iodine combine with
most elements, forming fluorides, chlorides, bromides, and
iodides ; this group of elements is called the halogen group,
and their compounds are often termed halides.

3. Oxygen and sulphur also combine with most elements,
and their compounds are named oxides and sulphides. The
comparatively rare elements selenium and tellurium form
similar compounds, named selenides and tellurides.

A very numerous and important class of compounds con-
sists of those in which oxygen is combined partly with
hydrogen, partly with another element. These compounds
can be divided into two distinct classes, according to their

ACIDS AND BASES 57

behaviour in aqueous solution. Members of both classes
are ionised, but they yield different ions according to the
class to which they belong. An example of the first class
is the compound H — O — Cl, known only in solution in
water, for it decomposes when an attempt is made to free it
from water. The aqueous solution is only slightly ionised,

+

but the Jons present are H and O — Cl. The hydro-
gen may be displaced by metals, forming " salts," which
are also ionised, and indeed much more completely than
H— O— Cl. Thus we have K— O— Cl, Ca=(O— Cl)2,

+
and other similar salts, which are ionised in solution to K

+ +

and O — Cl, and to Ca and (O — Cl)2 respectively. Such
hydroxides are named acids.

It appears, however, that elements which form this class
of hydroxide are, as a rule, incapable of retaining in com-
bination many hydroxyl groups at a time ; hence compounds
of this nature are generally mixed oxides and hydroxides.
It might, for example, be imagined that triad nitrogen
should be capable of retaining in combination three

,O— H
hydroxyl groups, to form H — O — N<f ; but the

\0-H

compound is unstable, and loses water, giving a mixed
hydroxide and oxide, H — O — N=O. The ions in this

case are H and O — N=O. Another similar instance is
that of sulphuric acid ; as it contains hexad sulphur, it
might be supposed that the corresponding hydroxide of
sulphur would be S(OH)6; but by loss of two molecules

H-O O

of water, sulphuric acid has the formula /

H— O/

as already shown. Its ions are H, H, and SO4, or some-

+
times H and HSO4. The salts of these acids are respec-

58 MODERN CHEMISTRY

lively M— O— N=O and (M— O)2=SO4, where M'
stands for any monad metal.

Nomenclature of Compounds. — The nomencla-
ture of this class of bodies is due to a committee of which
Lavoisier was a member. After his discovery of the true
nature of oxygen, he was led, not unnaturally, to ascribe to
it the chief function in the formation of compounds, and the
acids and salts were named without introducing any syllable
to signify that oxygen was one of the constituents. In
general, the best known or the first discovered acid was
given a name terminating in " ic," such as " chloric," " sul-
phuric," "nitric." The salts of these acids were termed
"chlorates," "sulphates," and "nitrates." The acid con-
taining one atom of oxygen less was named with the final
syllable " ous," thus : " chlorous acid," " sulphurous acid,"
"nitrous acid;" and the salts were termed " chlorites,"
"sulphites," and "nitrites." Acids containing still less
oxygen were named with the prefix " hypo," thus :
" hypochlorous acid " and " hypochlorites ; " and acids and
salts containing more oxygen than those which had names
terminating in " ic " and "ate" were distinguished by the
prefix "per," thus: "perchloric," " persulphuric " acids,
forming " perchlorates " and " persulphates." This no-
menclature is still retained. It is illustrated in the table
which follows : —

Hypochlorous acid, . . . HOC1.

Chlorous acid, . . . HOC1O.

Chloric acid, . . . HOC1O2.

Perchloric acid, .... HOC1O3.

Potassium hypochloritc, . . KOC1.

chlorite, . . KOC1O.

chlorate, . . KOC1O,.

perchlorate, . . KOCIO^.

The second class of hydroxides is named " hydroxides."
Members of this class, however, yield ions, one of which is

ACIDS AND BASES 59

always hydroxyl, OH. As examples, we may select
sodium hydroxide, Na— O— H, and calcium hydroxide,
Ca=(O — H)0. Here solutions of these compounds in

+ + +

water contain the ions Na and OH, and Ca and (OH)2.
Such hydroxides are termed bases ; but the name is also
indiscriminately applied to oxides when they unite with
water to form bases. Thus CuO and Cu(OH)2 are each
termed bases.

The same elements may sometimes form bases and acids,
according to the valency. The element chromium is an
instance. Chromous oxide has the formula Cr^O, cor-
responding to the chloride Cr=Cl.? ; the hydroxide is ana-
logous with the chloride, and has the formula Cr=(OH)2.
But there is also an oxide, CrOg, where chromium is a
hexad ; the hydroxide is not known, but the acid is like

H-O O

sulphuric acid in formula, viz., /Cr/ . This is an

H-O/ ^o

instance of the rule, of very wide application, that the
character of a compound is influenced both by the nature of
the elements contained in it, as well as by its structure and
the valency of these elements.

Sulphur, and in a less degree selenium and tellurium,
resemble oxygen in forming salts of nature similar to these
described, as well as acids and bodies analogous to hydrox-
ides. The nomenclature follows that of the oxides, except
that the syllables "sulpho" or " thio " are interposed for
the sulphur compounds. Thus we have a carbonate,
K0CO3, and a sulpho- or thiocarbonate, K2CS3. In the
somewhat rare cases where selenium or tellurium play a
similar part, the words " selenio- " or " telluric- " are
interposed. Compounds analogous to the hydroxides are
termed " hydrosulphides," " hydroselenides," or " hydro-
tellurides."

4. Compounds of nitrogen, phosphorus, arsenic, and
antimony, with other elements, are termed nitrides^ phos-

60 MODERN CHEMISTRY

arsenides^ and antimonides . And just as double
oxides of hydrogen and other elements exist, so too nitrides
of hydrogen and other elements are known. The compound
of nitrogen and hydrogen, ammonia, which has the formula
NH3, unites with acids, forming salts. For example,
ammonium chloride, NH4C1, is produced by the direct
union of ammonia, NH3, with hydrogen chloride, HC1,
if a trace of moisture is present. In aqueous solution it

+

undergoes partial ionisation, and the ions are NH4 and Cl.
In this it resembles sodium chloride, NaCl, and the name
"ammonium" has been devised to exhibit this similarity.
When ammonium chloride or similar compounds are formed
by the union of ammonia with acids, it is believed that the
nitrogen atom changes its valency from triad to pentad,

^H2

thus: H-N=H2 and Cl-N^f .

^H

.2

Even in " substituted ammonias," this property of com-
bining with acids is distinctive. For example, copper
chloride, CuCl9, unites directly with ammonia, giving
Cu=(NH2)22HCl or Cu=(NH3)2Cl2. It will be seen
that the atom of dyad copper has replaced two atoms
of monad hydrogen in two molecules of ammonium
chloride.

It is possible, too, for a group of elements to replace the
hydrogen of ammonia, just as it is possible for a group to
take the place of the hydrogen of water. Referring back
to the formula of sulphuric acid, H2SO4, it is plain that it
may be written SO2=(OH)2, and regarded as two mole-
cules of water, in which two atoms of hydrogen have been
replaced by the dyad group, SO2. This is identical with
the structural formula already given on p. 54. So, too,
with nitrogen; the compound SO2=(NH0)2 is also known,
and it may be regarded as derived from two molecules of
ammonia by the replacement of two atoms of hydrogen by
the dyad group, SO9".

CARBIDES AND SILICIDES 61

Compounds of phosphorus, arsenic, and antimony after
this pattern are not known.

5. Compounds of carbon and silicon with other elements
are termed carbides and silicides. The carbides are extra-
ordinarily numerous owing to the power possessed by carbon
of forming compounds in which two or more atoms of car-
bon are in combination with each other. We have seen
that a molecule of hydrogen consists of two atoms ; a mole-
cule of oxygen also consists of two atoms ; but we know of
no compounds of oxygen in which more than three atoms
of oxygen are in combination with each other. With
carbon, however, the case is different. Considering
the combination with hydrogen alone, we have not

H H

only CH4, but H3C-CH3, H8C-C-C-CH8, in all

H H

of which the element carbon is a tetrad, being either in
combination with hydrogen or with another atom of carbon.
And these compounds may have their hydrogen replaced
by other elements ; for example, an atom of chlorine may
take the place of an atom of hydrogen in any one of these
compounds, or one or more atoms of hydrogen may be re-
placed by groups of hydroxyl, — OH, or two atoms of
monad hydrogen may be replaced by an atom of dyad
oxygen, or three atoms by one of triad nitrogen, and so on.
This makes the chemistry of the carbon compounds very
complicated, but at the same time it affords the means
whereby the numerous constituent compounds of the tissue
of plants and animals can be built up, for they consist, for
the most part, of carbon compounds, containing at the same
time other elements in combination with the carbon atoms.
This branch of chemistry is commonly termed " Organic
Chemistry," and treated separately.

6. Many elements of the metallic class, such as iron,
lead, copper, sodium, &c., form compounds with each
other. These compounds are usually named "alloys;"

62 MODERN CHEMISTRY

but it must be mentioned that the name " alloy " is often
applied to solid mixtures of the metals where no actual
compound exists.

Such are the classes into which chemical compounds may
be divided. In a short sketch like the present, it is, of
course, impossible to do more than consider a few of these
compounds, and those will be chosen which are best adapted
to illustrate the nature of the various groups, They will be
considered in the second volume.

CHAPTER V

Methods of Determining the Equivalents of the
Elements — Of Ascertaining their Molecular
Weights — Allotropy. •

THE meaning of the word " equivalent " has already been
explained on p. 15, and we shall now consider how the
equivalent of an element may be determined. As already
stated, some compound of the element is analysed, prefer-
ably one with hydrogen, oxygen, or chlorine, and the
weight of the element which is in combination with, or
which replaces 8 parts by weight of oxygen, is termed
the equivalent of the element. But it is seldom that a
direct method of estimating the equivalent can be practised,
for it is not always possible to obtain a compound of the
element with hydrogen, or to deprive its oxide of oxygen,
or its chloride of chlorine. In fact, each element has to-
be specially studied, and a method devised which will lead
to the required information. It is, above all, necessary
that the compounds dealt with shall be pure — that is, that
they shall not contain any other elements than those which
it is desired to estimate, and that their composition shall be
definite. For instance, if it were desired to find the equiva-
lent of barium by estimating the proportion of chlorine in
its chloride, it would be essential to obtain barium chloride
free from the very similar elements calcium and strontium,
and it would also be of the first importance to make sure
that in weighing the chloride, the specimen should be free
from water adhering to the powdered substance.
63

64 MODERN CHEMISTRY

Methods of Determining the Equivalents of
Elements. — It is not always necessary to determine both
constituents of the compound ; for example, the ratio of
silver to chlorine can be found by dissolving a known
weight of pure silver in nitric acid, and then adding to the
solution some soluble chloride, such as hydrogen chloride ;
silver chloride is then precipitated thus : —

AgNO3. Aq. + NH4CLAq. - AgCl + NH4NO3. Aq.

The silver chloride is collected on a filter, thoroughly
washed, and after being dried, weighed. The Belgian
chemist, Stas, working in this way, obtained from 108.579
grams of silver 144.207 grams of silver chloride. The
relation between the atomic weight of oxygen, taken as
the standard and placed equal to 16, and the formula-
weight of silver chloride was ascertained by heating to
redness 138.789 grams of silver chlorate, 2AgClO3 =
2AgCl + 3O9; the weight of the residual silver chloride
was 103.9795 grams, and that of the oxygen evolved
taken as difference is 34.8095. The proportion —

Oxygen Silver chloride Q Formula weight

lost. remaining. of AgCl.

34.8095 : 103.9795 :: 48 : i43-38l7

gives the formula weight of silver chloride. The pro-
portion of silver it contains is found by the equation —

144.207 : 108.579 :: i43-38l7 : TO7-9583

Subtracting from 143.3817 the weight of the silver it
contains, 107.9583, the remainder is the atomic weight
of chlorine, which, for reasons already given, is identical
with its equivalent, namely, 35.4234; and 107.96 is the
equivalent of silver.

Knowing these facts, the atomic weight of, say, barium
may be determined by dissolving a known weight of its
chloride in water, and adding to the solution a solution

ATOMIC WEIGHTS 65

of silver nitrate, so as to obtain a precipitate of silver
chloride, which can be weighed, and from it the weight
of the chlorine in the barium chloride deduced. Sub-
tracting this from the weight of the barium chloride taken,
the remainder is the equivalent of barium. To determine
whether or not this number is identical with its atomic
weight, a determination of its specific heat must be made,
as described on p. 14.

In some instances the process is a more direct one. To
determine the equivalent of nickel, a weighed quantity of
the metal has been heated in oxygen, and the gain in
weight noted. Then, as this weight is to the weight of
nickel taken, so is the equivalent of oxygen to that of
nickel.

These examples will suffice to give a general idea of the
processes used in determining atomic weights, though, as
before stated, each element requires special treatment, and
the selection of the best method is often a very difficult
task. It is usual, moreover, to make determinations by
several methods, if that be possible, so as to avoid any
permanent source of error. Many observers, too, have
made such determinations, and it is not always easy to
eliminate a personal element from the results which they give.
A committee of the German Chemical Society has recently
published a table of atomic weights, reproduced below (with
a few alterations and additions), in which the last digit of
each number may in all probability be accepted as correct.
A second column is added, containing the atomic volumes
of the elements, so far as they are known. They represent
the volumes in cubic centimeters occupied by the atomic
weight of the element taken in grams, thus — 197.2 grams
of gold occupy 10.2 cubic centimeters. As the elements
expand on rise of temperature, these results are not always
comparative, but at present they are the best that can be
obtained.

66 MODERN CHEMISTRY

Table of Atomic Weights and Atomic Volumes.

Atomic Atomic

Weight. Volume.

Aluminium . Al 27.1 10. i

Antimony. . Sb 120 17.9

Argon . . A 39.9 32.9

Arsenic . . As 75 13.3

Barium . . . Ba 137.1

Beryllium . . .Be 9.1 4.3

Bismuth . . Bi 208.5 2T.2

Boron . . . B n.o 4.1

Bromine . . Br 79-96 25.1

Cadmium . . . Cd 112 13.0

Caesium , . Cs 133

Calcium . . Ca 40 25.3

Carbon . . C 12.00 3.4

Cerium . . Ce 140 20.8

Chlorine . . . Cl 35-45

Chromium . . Cr 52.1 7.7

Cobalt . . .Co 59.0 6.7

Copper . . . Cu 63.6 7.1

Erbium . . . Er 166 ?

Fluorine . . F 19

Gadolinium . . Gd 156

Gallium . . . Ga 70 u.8

Germanium . . Ge 72

Gold . . . Au 197.2 10.2

Helium . . . He 4

Hydrogen. . . H 1.007

Indium . . .In 114 25«7

Iridium . . Ir I93-c 8.6

Iodine . . .1 126.85 25.7

Iron . . . Fe 56.0 6.6

Krypton . . . 81.5 37.8

Lanthanum . . La 138 22.9

ATOMIC WEIGHTS 67

Atomic Atomic

Weight. Volume.

Lead . . . Pb 206.9 18.2

Lithium . . .Li 7.03 11.9

Magnesium . . Mg 24.36 13.3

Manganese . . Mn 55.0 7.7

Mercury . . . Hg 200.3 T4*8

Molybdenum . . Mo 96.0

Neodymium . . Nd 143.5

Neon . . . Ne 20

Nickel . . Ni 58.7 6.7

Niobium . . . Nb 94 14.5

Nitrogen . . N 14-04

Osmium . . Os 191 8.9

Oxygen . . . O 16.000 (standard).

Palladium . . . Pd 106 9.3

Phosphorus . . P 31.0 17.0

Platinum . . . Pt 195.2 9.1

Potassium. . . K 39«I4 45«5

Praseodymium . . Pr 141

Rhodium . . . Rh 103.0 9.5

Rubidium . . . Rb 85.4 56.3

Ruthenium . . Ru 101.7 9.2

Samarium . . . Sm 150

Scandium . . Sc 44

Selenium . . . Se 79.1 18.5

Silicon . . .Si 28.4 11.4

Silver . . . Ag 107.93 10.3

Sodium . . . Na 23.05 23.7

Strontium . . Sr 87.6 34.5

Sulphur . . . S 32.06 15.7

Tantalum . . . Ta 183 17.0

Tellurium. . . Te 127.6 20.3

Thallium . . . Tl 204.1 17.2

Thorium . . . Th 232 29.8

Thulium . . . Tu 170?

Tin . . . . Sn 1 19.0 16.2

68 MODERN CHEMISTRY

Atomic Atomic

• Weight. Volume.

Titanium . . . Ti 48.1

Tungsten . . . W 184 9.6

Uranium . . U 240 13.0

Vanadium. . .V 51.2 9.3

Xenon . . .X 128 35.9

Ytterbium. . . Yb 173

Yttrium . . . Y 89

Zinc . . . Zn 65.4 9.5

Zirconium . . Zr 90.6 21.9

Molecular Weights of the Elements. — The

molecular weights of some of the elements have been
successfully determined ; in certain cases by their density
in the gaseous state, in others by the lowering of the
vapour-pressure of mercury, caused by the presence of a
known weight of a dissolved metal, and again in others by
the depression of the freezing-point of certain metals, caused
by the presence of others in known amount. These will
be considered in their order.

(a) Vapour- densities. — For reasons already explained
on page 13, a molecule of oxygen is believed to contain two
atoms, and inasmuch as the equivalents of most elements
have been determined with reference to oxygen, by analysis
or by synthesis of their oxides or of their chlorides, and
as the ratio of the equivalent of chlorine to that of oxygen
has been very accurately determined, it has been agreed to
refer the atomic weights of the elements to the standard
of oxygen instead of to that of hydrogen. But the atomic
weight of oxygen is assumed as 16, and the same standard
is applied to the densities of gases; instead of referring them
to the standard of H = I, they are referred to O = 16. To
find the molecular weights, the number expressing the
density must be doubled in order to compare with the
molecular weight of oxygen, which is 32.

Hydrogen. — The density referred to this standard is

MOLECULAR WEIGHTS 69

i. 006 or 1.007. There is not yet an absolute certainty,
but it is clear that the molecular weight of hydrogen must
be approximately 2, i.e. the molecule is di-atomic.

Nitrogen. — Lord Rayleigh found the density of nitrogen
to be 14.001 ; its molecular weight is therefore 28, and its
formula N2.

Oxygen. — Taken as 16; formula O2. As these gases
keep their relative densities up to a temperature of 1700°,
it is to be presumed that they all remain diatomic, for it is
much more likely that no one of them dissociates than that
all dissociate to an equal extent on rise of temperature.
The case is different with fluorine, chlorine, bromine,
and iodine. The density of fluorine at atmospheric tem-
perature is 18.3; the theoretical density for F2 is 19. It
follows, therefore, that fluorine must consist of a mixture
of monatomic and diatomic molecules. Now, 19 is the
molecular weight of FI, for the atom and the molecule are
identical, and 38 that of F2 ; and the gas must contain
x molecules of FI+(I— x) molecules of F2. Hence,
19x4- 38(1 — x) = 18.3 x 2 ; and x = 0.073, *•*• 'in every
1000 molecules of the gas there are 73 molecules of Fj
and 927 molecules of F2.

Chlorine at 200° was found to have the density 35.45,
the same as its atomic weight, but at 1000° the density
was 27.06, and at 1560° 23.3. At low temperatures,
therefore, the formula of chlorine is C12, but at 1560° the
gas consists of 61 per cent, of molecules of CIr Similar
results have been found for bromine, and for iodine, which
also has the formula I0 at low temperatures, the density
was found to be 63.7, corresponding to the molecular
weight 127.4 at 1500° under low pressure ; for reducing
the pressure also increases dissociation. As the atomic
weight of iodine is 126.85, tne £as at I5°°° consists almost
entirely of molecules of Ir

Thallium has been weighed as gas at 1730° ; the density
was 206.2, a sufficient approximation to 204.1 to warrant
the conclusion that its molecule is diatomic.

70 MODERN CHEMISTRY

Bismuth at 1640° gave the density 146.5, showing, as
its atomic weight is 208.5, a Partial dissociation from Bi0
to Bi-p

Phosphorus and arsenic give densities which indicate
the presence in their gases of more complicated molecules.
At 313° the density of phosphorus gas is 64, and there
is a gradual decrease with rise of temperature, until at 1708°
the density is 45.6. As the atomic weight of phosphorus
is 31.0, the density 62 would correspond to the existence
of molecules of P4, while at 1708° there must be a con-
siderable admixture of molecules of a smaller complexity,
probably P£. Arsenic gas had the density 154.2 at 644°,
and 79.5 at 1700°; the atomic weight of arsenic being 75,
the density 150 would correspond to the formula As4, and
at 1700° the molecules are almost all As2, only a small
admixture of molecules of As4 remaining undecomposed.
The density of antimony gas, 141.5 at 1640°, implies the
presence of some molecules of Sb4 among many molecules
of Sb2, for the atomic weight is 120.

The elements sulphur, selenium, and tellurium show
signs of even greater molecular complexity. Dumas found
the density of sulphur gas at 500° to be 94.8 ; now, the atomic
weight of sulphur is 32.08, and 96 is 32 x 3 ; hence, it
was for long supposed that a molecule of gaseous sulphur
consisted of 6 atoms ; but it has been recently found that
at 193°, of course under a very small pressure, 2.1 mms.
(for the boiling-point of sulphur at normal pressure is 446°),
the density reached the high number 125.5 5 now> 32 x 4 is
128, and it must be concluded that the molecular weight
of sulphur in the gaseous state is 256, and its formula at
low temperatures Sg. At 800° its formula is S9, and at
1719° the density 31.8 was found, showing no sign of
further molecular simplification. Selenium, of which the
atomic weight is 79.1, has the density 1 1 1 at 860°, imply-
ing some molecular complexity, and at 1420° the density
is reduced to 82.2, corresponding to the formula Se.2 ;
and tellurium, at about 1400°, has the gaseous density 130;

MOLECULAR WEIGHTS 71

it appears, therefore, to consist of molecules of Te9, since
its atomic weight is 127.6.

These examples show that the molecules of many ele-
ments in the gaseous state are more or less complex. It is
probable that sulphur, selenium, and tellurium would exist
as octo-atomic molecules could the temperature be suffi-
ciently reduced ; even with sulphur at its boiling-point under
normal pressure, the temperature is so high that many of
these complex molecules are already decomposed. Proba-
bility is also in favour of the supposition that elements of
the phosphorus group, phosphorus, arsenic, antimony, and
possibly bismuth, have molecules consisting of 4 atoms ;
these too dissociating with rise of temperature into di-atomic
molecules. Oxygen, nitrogen, and hydrogen consist of
di-atomic molecules, no sign of dissociation having been
remarked even at the highest attainable temperatures; but
fluorine, though consisting mostly of di-atomic molecules,
contains some mono-atomic ones ; and chlorine, bromine,
and iodine, though probably CJ9, Br2, and I2 at low
temperatures, dissociate into molecules identical with their
atoms if the temperature is sufficiently raised. The fact of
reduction in the molecular complexity of the molecules of
elements prepares us for the existence of elements which in
the gaseous state are already mono-atomic ; and many such
are known.

Mono-atomic Cad-
elements. Sodium. Potassium. Zinc. mium. Mercury.
Gas-density 12.7 18.8 3415 57-oi 100.94
Temperature Red heat Red heat 1400° 1040° 446° and 1730°
Atomic weights 23.05 39-!4 65.4 112.0 200.3
Density x 2 25.4 37.6 68.3 114.02 201.88

The presumption from these numbers is that the
elements are all mono-atomic. It must be remembered
that their specific heats all point to the atomic weights
given.

There is, however, another argument for the mono-
atomicity of gaseous mercury. On the assumption of the

72 MODERN CHEMISTRY

" kinetic theory of gases," that the pressure of a gas on the
walls of the vessel containing it is due to the bombardment of
the sides by repeated and enormously numerous impacts of
the molecules, it can be calculated that the amount of heat
necessary to raise the temperature of the molecular weight
expressed in grams of an ideal gas the molecules of which
are supposed to be hard smooth elastic spheres, must be 3
calories, provided the gas be not allowed to expand. If,
however, it be allowed to expand, it will cool itself, and
more heat must be added to restore the temperature ; this
extra amount of heat is two additional calories. To heat
the molecular weight of the gas in grams through I °, allow-
ing it to expand at constant pressure, requires therefore 5
calories. The " molecular heat at constant volume" is
thus 3 calories; the "molecular heat at constant pressure"
is 5 calories. The ratio between the two is 3 : 5, or
I : 1.66. This has been found to be the case for mercury
gas, the mono-atomicity of whose molecule is proved on
other grounds ; and the inactive gases of the atmosphere,
helium, neon, argon, krypton, and xenon, exhibit the same
ratio between their atomic heats. It therefore follows that
the atoms of these gases are also identical with their mole-
cules ; and that their atomic weights are to be deduced from
their densities by doubling the numbers representing the
latter. Confirmatory of this view, the ratio between the
molecular heats of oxygen, hydrogen, nitrogen, and gases
which are known to be di-atomic, like NO, CO, &c., is as
5 : 7 or i : 1.4. Such gases require more heat to raise
their temperature than an equal number of molecules of the
mono-atomic gases do ; the reason is, that the heat applied
to di- or poly-atomic gases is used, not merely in transport-
ing the atoms from place to place and raising pressure by
causing them to bombard the walls of the containing vessel,
but some heat is required to cause the atoms to move within
the molecule, in some rotatory or vibratory manner ; and
consistently with this it has been found that gases consisting
of a greater number of atoms in the molecule require still

MOLECULAR WEIGHTS 73

more heat to raise the temperature of weights proportional to
their molecular weights ; in other words, their molecular
heats at constant volume are higher the greater the number
of atoms in the molecule.

For these reasons the densities of the inactive gases must
be multiplied by 2 to obtain their atomic weights. The
data are : —

Helium. Neon. Argon. Krypton. Xenon.

Densities 2 10 20 41 64
Atomic and

molecular weights 4 20 40 82 128

(£) Lowering of Freezing-Point, or Lowering of
Vapour-Pressure of Solvent. — The molecular weights
of some of the elements have been determined by Raoult's
method, either by the lowering of the vapour-pressure
of mercury, or by the depression in the freezing-point
of some other metal or solvent in which the element has
been dissolved. Lithium, sodium, potassium, calcium,
barium, magnesium, cadmium, gallium, thallium, manganese,
silver, and gold appear to be mono-atomic, while tin, lead,
aluminium, antimony, and bismuth show tendency in con-
centrated solution to associate to di-atomic molecules.
These results were obtained by measuring the lowering of
vapour- pressure of mercury produced by known weight of
the metals named. By measurement of the depression in
the freezing-point of tin, in which metals were dissolved,
zinc, copper, silver, cadmium, lead, and mercury appeared
to be mono-atomic, while aluminium was found to be di-
atomic. These results, however, are not to be regarded
with the same confidence as those obtained by means of
measurements of the vapour-density, for it is not certain
whether the molecular weight of the solvent should be taken
as identical with its atomic weight. All that can be certainly
affirmed is, that the molecular weights of the elements which
have been placed in the same class above correspond to
formulas with the same number of atoms in the molecule.
Thus, if zinc is mono-atomic, so is cadmium ; if di-atomic,

74 MODERN CHEMISTRY

cadmium has also a di-atomic molecule ; and similarly with
the rest.

A method has also been devised, depending on the capil-
lary rise of liquids in narrow tubes, by means of which it is
possible to estimate the molecular complexity of liquids.
This method is applicable to only a few elements ; but by
its use it has been found that in the liquid state bromine
consists chiefly of di-atomic molecules mixed with a few
tetra-tomic molecules ; and that phosphorus in the liquid,
as in the gaseous condition, forms molecules corresponding
to the formula P4.

Allotropy. — Closely connected with this question is
the phenomenon of allotropy. This word, which signifies
" other form," is applied to the existence of elements in
more than one condition. Thus phosphorus, which is
usually a yellow, waxy substance, with a low melting-
point, changes its appearance when heated, and becomes
converted into a red amorphous powder, insoluble in the
usual solvents for phosphorus, such as carbon disulphide,
and melting at a much higher temperature than the yellow
variety ; moreover, the red form is much less easily in-
flamed than the yellow form. These two forms are said
to be allotropic, and the element is said to display allo-
tropy.

The elements which display allotropy are : — carbon,
silicon, tin, phosphorus, arsenic, antimony, oxygen, sulphur,
selenium, iridium, ruthenium, rhodium, silver, gold, and
iron. These will be considered in their order.

Carbon. — Diamonds, as was discovered by Lavoisier,
yield on combustion nothing but carbon dioxide ; their
identity with carbon was thus proved. When pure, they
are colourless ; they are the hardest of all known sub-
stances, and possess a density of 3.514 at 18°. When
heated in absence of air in an electric arc, a diamond
changes to a coke-like black substance. Diamonds of
any appreciable size have not been formed artificially, but
minute diamonds have been made by Moissan by dissolving

ALLOTROPY OF CARBON 75

carbon in molten iron heated to its boiling-point in an
electric furnace, and then suddenly cooling the iron by
plunging it into molten lead; the external surface of the
iron solidifies, and encloses a molten interior. As iron
possesses a greater volume in the solid than in the liquid
state, the molten iron, containing carbon in solution, when
it solidifies is under great pressure, for it is confined and
hindered from expanding by the crust of solid iron ; under
this pressure the carbon separates out in the liquid form,
and in solidifying crystallises in octahedra with curved facets
characteristic of natural diamonds. If, on the other hand,
the iron is allowed to cool without any device to compress
the interior, the carbon crystallises out in the form of
graphite or plumbago, or, as it is sometimes termed, " black-
lead." This variety of carbon is also found native ; it
forms hexagonal plates, is soft, and is slippery to the touch.
Lastly, many compounds of carbon when heated to redness
decompose, and leave the carbon in an amorphous or non-
crystalline form. Varieties of these are gas- carbon, deposited
in the necks of gas-retorts ; oil- coke, left as a residue after
the distillation of certain oils ; sugar-charcoal, the residue
on heating sugar in absence of air ; and wood-charcoal, the
product of the distillation of wood. All of these are black,
more or less hard substances. When heated to whiteness
in an electric arc, they are transformed into graphite. They
all contain a trace of hydrogen, from which they can be
freed by heating to redness in a current of chlorine. At
the temperature of the electric arc, carbon volatilises with-
out fusion and condenses as graphite ; it is only when it
is heated under pressure, as described, that it can be made
to melt.

Silicon. — This element exists in three forms, two of
them crystalline, the third amorphous. The amorphous
modification when dissolved in molten zinc or aluminium
crystallises out in either black lustrous tablets resembling
graphite or in iron-grey prisms. It is not known what
circumstances determine the formation of the one or the

76 MODERN CHEMISTRY

other form. Silicon melts at a bright red heat, and can
be cast into rods ; they have the graphite-like crystalline
form.

Tin. — This metal, when kept at a low temperature —
about —30° — changes to a grey powder. On heating the
powdery modification to above 20°, it is converted back
into ordinary metallic tin, the more quickly the higher the
temperature. If the powder be left in contact with ordinary
tin at atmospheric temperature, the metal is slowly changed
into its allotropic modification, and articles of tin fall to
pieces.

Phosphorus. — Three forms are known for phosphorus.
The first, or ordinary form, is a waxy solid, melting at
44.4°. It is soluble in carbon disulphide, and crystallises
from it in rhombic prisms. It is luminous in the dark
in presence of air, but if the pressure of the air be raised
it ceases to shine ; it is also non-luminous in oxygen. It
is very easily inflamed, and burns to its oxide, P2^s* ^ ls
poisonous when swallowed. The liquid obtained by melting
it is nearly colourless. When this variety is heated to 240°
in a vessel from which oxygen is excluded, it changes to
a red substance, generally termed amorphous phosphorus.
This body is insoluble in carbon disulphide and the other
solvents which dissolve ordinary phosphorus. It is not lumi-
nous in the dark, and is not easily oxidisable. When heated
to a temperature higher than 240°, it volatilises and condenses
to ordinary phosphorus, and if air be present it takes fire.
It is soluble in lead, and when the molten lead cools it
crystallises out in nearly black crystals. Indeed, its colour
depends on the temperature at which it is formed. If
produced at 260°, it is deep red, and has a glassy fracture ;
at 440° it has a granular fracture and is orange; at 550°
it is grey, and it fuses at 580°, and on solidifying it
forms red crystals. It is possible, though not probable,
that a mixture of several allotropic forms is the cause of all
these changes.

Arsenic. — When arsenic is distilled, it passes directly

ALLOTROPY OF OXYGEN 77

from the gaseous to the solid state on condensing. The por-
tion which cools most quickly is a black powder ; that
which condenses in the warm part of the tube has a grey
metallic lustre. The black variety can be converted into
the crystalline metallic variety on heating to 360°. When
arsenic is heated in an indifferent atmosphere under great
pressure, its boiling-point is raised above its melting-point,
and it melts ; on solidification, it forms the metallic variety.

Antimony. — The usual form of antimony is a white
brittle metal with a faint bluish tinge. If deposited from
a strong solution of its chloride by electrolysis, a grey
powdery deposit is formed, which has the curious property
of exploding when heated or struck ; it then changes into
the metallic variety. It has a lower density than the
ordinary antimony.^

Oxygen. — The allotropic variety of oxygen is named
ozone ; it was discovered by Schoenbein, and is obtained by
causing a shower of minute electric sparks (the " silent
electric discharge") to pass through oxygen, preferably
cooled to a low temperature. One of the best forms of
" ozoniser " is a tube about I cm. in diameter, partially
evacuated, and traversed by a wire from end to end ; this
tube is contained in a wider one, and the space between the
two tubes contains a set of metallic annuli, connected
together by a wire. Oxygen is passed slowly through the
space between the two tubes, while the two wires are
connected with the secondary terminals of a coil ; sparks
pass through the inner glass and the space between the two
tubes. On first passing the current the oxygen expands, but
almost at once contraction ensues, and ozone issues at the
further end of the tube. It is not possible, in dealing with
ozone, to use indiarubber connections of any sort, for the
rubber is at once attacked. Ozone is also formed when
phosphorus slowly oxidises in moist air ; when the vapour of
ether or benzene is stirred with a hot glass rod in presence of
air ; when sulphuric acid acts on barium dioxide or potas-
sium permanganate, or when sulphuric acid is electrolysed.

78 MODERN CHEMISTRY

It is also produced in large amount when fluorine comes into
contact with water.

Its name refers to its most striking property — its strong
disagreeable smell. It condenses when cooled by liquid air
to a dark blue liquid, which is very explosive ; and its gas,
when seen in a long tube, has also a blue colour ; it shows
characteristic spectral bands. The blue liquid boils at
— 1 06°, whereas the boiling-point of oxygen is-i82°. Liquid
oxygen is also blue, but has quite a pale tint. When heated
to 250°, ozone is re-convened into ordinary oxygen; but
oxygen cannot be transformed into ozone by heat alone.
Ozone is a much more active body than oxygen ; it liber-
ates iodine from potassium iodide (aKI.Aq + O3 + H2O =
2KOH. Aq + L, 4- O9) ; it oxidises metallic silver and
mercury (Hg + O3 = HgO + O0) ; and it changes lead sul-
phide into sulphate (PbS + 4O8=*PbSO4+4O2). When
passed through a solution of hydrogen dioxide, oxygen
is evolved (HL,O2.Aq + O3 = H2O.Aq+ 2O2) ; and it
bleaches indigo and other colouring matters.

Its density is 24, that of oxygen being 16 ; whence its
formula is O3. Its rate of diffusion into air bears to that
of chlorine, of which the density is 35.47, the ratio of
/24 • /35«5 — another proof of its density. When oxygen
is converted into ozone, the portion which is changed con-
tracts in the proportion 3:2; and conversely, when ozone
is heated and converted into ozone, that portion of the gas
which consists of ozone increases in volume from 2 : 3.
All these proofs demonstrate that the formula of ozone

isOy

Ozone is a poison ; it excites coughing, and in large
quantity asphyxiates, the blood becoming venous. It is
very doubtful whether ozone has been found in the atmos-
phere, except, perhaps, after a thunderstorm.

Sulphur. — The allotropy of gaseous sulphur has already
been alluded to ; that of liquid and of solid sulphur is no less
striking. When sulphur is melted, it forms a mobile, light
brown liquid. On raising the temperature, the liquid be-

ALLOTROPY OF SULPHUR 79

comes viscous, so much so, indeed, that the vessel contain-
ing it can be inverted without spilling the liquid ; and at a
still higher temperature it again becomes mobile, but has
a deep brown colour. On cooling, these changes are re-
versed. If viscous sulphur be poured into water, it hardens
to a substance resembling indiarubber ; this form, if kept for
some hours, falls into minute octahedral crystals. When
molten sulphur is allowed to cool slowly, it solidifies at 120°,
forming long monoclinic needles of a pale brown colour.
This variety is also deposited on evaporating a solution of
sulphur in ether or in benzene. These needles, on standing
for a few hours, become opaque and spontaneously fall into
minute rhombic octahedra. Octahedral crystals of a large
size may be produced by allowing a solution of sulphur in
carbon disulphide to evaporate spontaneously ; this variety
melts at 115°; its colour is bright yellow; it is in this
form that sulphur occurs native. Its density is 2.07,
whereas that of monoclinic sulphur is 1.97 at o°. Two
other varieties of sulphur are known. If sulphur vapour be
quickly cooled, it condenses in a dusty form, termed
" flowers of sulphur ; " this powder, if treated with carbon
disulphide, leaves an insoluble residue, distinct from all other
forms, which are all soluble in disulphide. And lastly, if
sulphur be produced in presence of water by the decompos-
ing action of water on sulphur chloride, or by the action of
hydrogen dioxide on hydrogen sulphide, the sulphur does
not separate, but remains in a state of " pseudo-solution " in
the water ; it can be precipitated on addition of salts such
as calcium chloride. It is thus evident that the chemistry
of the element sulphur is very complicated.

Selenium. — This element has three allotropic forms ;
when precipitated from selenious acid by sulphurous acid —
H2SeO3. Aq + 2H2SO3.Aq = Se + 2H2SO4. Aq + H2O—
it forms a red powder, soluble in carbon disulphide, and
crystallising therefrom in dark red crystals, a non-conductor
of electricity. Either the amorphous red variety or these
crystals, if kept at 210° for some time, change into a black

8o MODERN CHEMISTRY

crystalline variety, insoluble in carbon disulphide, and con-
ducting electricity on exposure to light. These varieties
also differ in density and melting-point.

Ruthenium, Rhodium, and Indium are grey-white
metals, hard and fusible only at a very high temperature.
They are insoluble in hydrochloric, nitric, or sulphuric
acid. On alloying them with zinc or lead, and then dis-
solving out the alloyed metal with acid, the ruthenium,
rhodium, or iridium is left as a black powder, exploding
when gently warmed, and going back into the ordinary
form of the metal.

Iron. — It has been known for many centuries that the
properties of iron are profoundly modified when it contains
a small percentage of carbon ; it is then termed steel.
Steel has a fine granular fracture, and is not fibrous like
pure wrought iron, or coarsely crystalline like cast-iron,
which contains a greater proportion of carbon than steel,
the latter containing from 0.8 to 1.9 per cent. When steel
is heated and then suddenly cooled, — an operation termed
" tempering," — it becomes very hard; this is due to a change
which takes place in the molecular structure of iron at 850°.
At that temperature its specific heat suffers a considerable
change ; and if the iron contains a small percentage of
carbon, the allotropic state persists after the cooling has
taken place, if produced sufficiently rapidly. The various
qualities of steel, elastic as in springs, hard as in razors,
brittle and extremely hard as in files, are due to admixture
of more or less of the allotropic modification with ordinary
iron, which is a comparatively soft metal.

Silver and Gold. — Several metals, among them silver,
gold, and platinum, when precipitated from aqueous solutions
of their salts by some reducing agent, such as sodium
formate, form apparent solutions of the metal in water.
That of platinum is grey, of silver blue or red, and of gold
purple. The colour, however, depends on the state of
division of the metal, and it may vary greatly with the
same metal. If the "pseudo-solution" of platinum is

PHASES 81

warmed, the metal is precipitated as a black powder, known
as " platinum-black." This substance readily absorbs
gaseous oxygen or hydrogen ; when heated, it is converted
into a grey powder, obviously finely divided ordinary plati-
num, termed "platinum sponge." On evaporating the
pseudo-solutions of silver or gold, the metal remains as a
coloured residue ; on warming or rubbing, it changes into
the ordinary metal.

These are the known cases of allotropy. In some cases,
as when the gases of ozone or sulphur are weighed, a direct
clue to the molecular weight, and therefore the cause of
isomerism, is revealed ; but in others, where the different
modifications are liquid or solid, there is no obvious means
of tracing the cause of the allotropy. Yet in some instances
a reasonable theory can be formed.

Phases. — We know that a liquid and its gas can exist
together at different temperatures, and that at high tem-
peratures the gas exerts a greater pressure than at low.
To each temperature corresponds a definite pressure. If
the temperature be named, it is termed the "boiling-point"
at that pressure ; if the pressure for any particular tem-
perature be alluded to, it is called the " vapour-pressure."
According to the molecular theory, the vapour-pressure is
reached when as many molecules leave the liquid surface in
unit time as return to it by the condensation of the gas.
There is a state of equilibrium ; but on raising the tem-
perature the equilibrium is disturbed, and more vapour is
given off to restore it. The gaseous state and the liquid
state are termed two phases of the same kind of matter,
and they can coexist at various temperatures.

When the pressure is reduced below a certain amount
— for water to 4.6 mms. — the boiling-point is lowered to
o°. At this temperature water usually freezes under a
pressure of one atmosphere. As the pressure is 4.6 mms.,
the freezing-point of the water will be at 0.007° above
zero ; for it is found that the freezing-point of water is
lowered by that amount for each rise of one atmosphere

VOL. i. F

82

MODERN CHEMISTRY

pressure ; and if the atmosphere pressure be removed, the
freezing-point will be raised. At this temperature, there-
fore, ice, water, and water-vapour are all in equilibrium
with each other and can coexist. The point is called the
" triple point." The states of water, ice, and steam can
be represented by a diagram.

B

PRESSURE

t

SOLID

L V

VAPOUR

A

TEMPERATURE

Let pressures be measured up the vertical line AB,
and temperatures along the horizontal line AC. The point
O corresponds to the temperature 0.007° and to the pres-
sure 4.6 mms. Along the line OLV (LV standing for
liquid-vapour) the liquid and the vapour can coexist ; it is
usually termed the " vapour-pressure curve." The line O
SL (solid-liquid) shows the alteration in the melting-point
of ice as the pressure rises ; its slope is greatly exaggerated
in order to make it visible. It shows that as the pressure is
raised the melting-point of ice becomes lower and lower.
Lastly, the line OSV (solid- vapour) indicates the coexist-
ence of the solid and the vapour phases ; the pressure is

PHASES OF WATER 83

below 4.6 mms., and the temperature below o°. The
regions shown correspond to those conditions of temperature
and pressure where the substance can exist as solid, as liquid,
or as gas. The dotted line WO represents the condition of"
a super-cooled liquid. It is possible to cool water below o° ;
for example, if it be pure and kept at rest, its vapour-
pressure is then greater than that of ice at the same tempera-
ture. It has not been found possible to heat ice above its
melting-point without its melting, but it is possible to cool
steam somewhat below its condensing temperature without
condensation occurring, as shown by the dotted line OP.
These states of relative instability have been called " meta-
stable." Shaking the water or introducing particles of
dust into the steam at once induce freezing or condensation ;
the water changes to ice or the steam condenses.

The condition of allotropy can be similarly represented.
JBut the problem is more complicated ; in the case of
sulphur, for example, there are two solid phases, the rhombic
and the monoclinic, besides more than one liquid phase. As
the two solid phases, the rhombic melting at 115°, and the
monoclinic melting at 120°, are well known, attention will
be confined to them.

If sulphur be allowed to crystallise from fusion, it assumes
the monoclinic form of long prisms. These crystals, however,
change spontaneously at the ordinary temperature, and in a
few hours fall into minute rhombic octahedra. At the
temperature 95.6°, however, this change no longer takes
place ; the two crystalline forms can coexist in presence of
each other without one form turning into the other. The
rhombic variety gives off vapour which of course exerts
pressure at that temperature and at lower temperatures at
which the rhombic variety is stable ; and the vapour-
pressure curve is indicated by the line PRV (rhombic-
vapour). This temperature is termed the "transition
temperature " for rhombic and monoclinic sulphur and
sulphur-vapour. It may be compared with the melting-
point of ice under 4.6 mms. pressure, which, it will be

84

MODERN CHEMISTRY

remembered, is 0.007°, where water, ice, and steam are in
equilibrium. But it differs inasmuch as it is possible to
heat rhombic sulphur above the transition-point P without
immediate change. PO is the vapour-pressure curve for
monoclinic sulphur, which melts at 120° ; and each of these
curves must meet in the transition-point P, for at that tempe-
rature both modifications can coexist. Below 95.6 the
monoclinic form is in the metastable condition, and the line

JLR

Pressure

95-6

//5° <2O°

Te mpe Tcutu. re

PMV, which is a continuation of the line OP, expresses
the vapour-pressure of the monoclinic variety below the
transition temperature. The rhombic variety above 95.6°
is in the metastable condition, and its vapour-pressure is
shown by the line PRV, the upper portion of which is
dotted. If the sulphur be compressed, the transition-point
rises, and the line PQ typefies this. At 120° there must be
another transition-point, for here rhombic sulphur, liquid
sulphur, and sulphur-vapour may exist in presence of each

PHASES OF SULPHUR 85

other. Now the melting-point of sulphur is raised by
pressure instead of being lowered, as in the case of water.
This is the more usual ; the lowering of melting-point of the
solid water depends on the fact that the density of ice is less
than that of water, but that of solid sulphur is greater than
that of molten sulphur. Hence the rise of the transition-
point along the line PQ. At O three curves meet : OP,
representing the vapour-pressure of monoclinic sulphur ; O
LV, the vapour-pressure of liquid sulphur; and OQ, the
effect of pressure in raising the melting-point of rhombic
sulphur. The lines PQ and OQ happen to meet at Q,
which is also a transition-point ; it lies at 131° ; and here
rhombic, monoclinic, and liquid sulphur can all coexist,
though the pressure is too high for vapour to exist along
with them. At higher temperatures and pressures mono-
clinic sulphur is incapable of existence. As we have seen,
the metastable states of sulphur are capable of existence for
some time. Rhombic sulphur can be heated to its melt-
ing-point, 115°, which lies above its transition tempera-
ture. At this temperature it and the liquid resulting from
its fusion are both in a metastable condition. And the
effects of pressure in raising the melting-point of rhombic
sulphur is shown by the dotted line RVQ, which is
continued in QLR (liquid-rhombic) at temperatures at
which the monoclinic variety is no longer capable of exist-
ence.

Although the allotropy of other elements has not been
so minutely studied as that of sulphur, it is certain that the
various conditions can all be represented in a similar manner.
For example, the transition-point of the grey and metallic
modifications of tin is 20° ; below that temperature metallic
tin is in the metastable condition ; if cooled sufficiently, it
may change spontaneously into the grey powder, but the
change may not take place, just as water may be kept
super-cooled without freezing. But just as the addition
of a crystal of ice to super-cooled water causes it to
crystallise, so the contact of grey tin below 20° with

86 MODERN CHEMISTRY

metallic tin induces the change ; the surface of the tin
becomes covered with spots like pimples, and, if time be
given, all the tin falls to powder. The change is the more
rapid, up to a certain point, the lower the temperature. If
the grey tin be raised in temperature above 20°, it is recon-
verted into metallic tin, the more quickly the higher the
temperature.

The yellow waxy condition of ordinary phosphorus, too,
appears to be a metastable condition, for if its temperature
is raised under pressure, red phosphorus is produced. On
the other hand, if red phosphorus be heated under ordinary
pressure, it volatilises and condenses as yellow phosphorus.
Nevertheless, at the very highest temperatures, the vapour-
pressure curves would indicate that yellow phosphorus is
the stable form.

We are still in the dark as to the precise reason of such
allotropic changes. From cases which can be investigated,
owing to the liquid or gaseous states of the allotropic modi-
fications, the cause would appear to consist in a greater or
less molecular complexity, but this is not proved for solids.
It is possible that the cause of allotropy is to be found, in
some cases at least, in a different arrangement of the mole-
cules in the solid, and this suggestion falls in with the fact
that allotropy often consists in different crystalline forms,
but it is also conceivable that a different crystalline form
may correspond with difference in molecular complexity
as well as with different molecular arrangement. Until
some method is discovered whereby the molecular weights
of solids can be determined, it is not probable that certainty
will be attained.

CHAPTER VI

Isomerism — Polymerism — Optical and Crystal-
lographic Isomerism — Stereo- Isomerism —
Tautom erism .

CLOSELY connected with allotropy is what is termed
isomerism. Attention was first called to the existence of
compounds with identical composition, so far as it could
be ascertained by analysis, but possessing different physical
properties, such as melting-point, boiling-point, and crys-
talline form, and different chemical properties, by Wohler
and by Liebig. Faraday, too, drew attention to a similar
phenomenon which has received the name of polymerism.
All food contains the elements carbon, hydrogen,
oxygen, and nitrogen, besides sulphur, phosphorus, and
other elements. The main constituents of food are starch,
which is the chief component of bread, and which is
devoid of nitrogen ; and albumen and allied bodies, of
which flesh mainly consists, which is rich in nitro-
gen; dried meat, indeed, contains from 10 to 12
per cent, of that element. During the passage of food
.through the system, the carbon and hydrogen are mainly
eliminated by the lungs as carbon dioxide, CO2, and water,
H2O ; while the largest portion of the nitrogen passes
away through the kidneys, in the form of a compound
named urea, of the formula CON2H4. Now Wohler
succeeded in 1827 in forming urea artificially by preparing
a compound of ammonia, NH3, with an acid termed cyanic
acid, HNCO, itself producible from the elements uhich
87

88 MODERN CHEMISTRY

it contains. On heating this compound, ammonium cyanate,
NH4NCO, to the temperature of boiling water, it under-
goes an " isomeric change/' Before such heating, if the
ammonium cyanate be warmed with a solution of caustic
potash, the smell of ammonia is at once apparent ; this is
the usual test for ammonium ions ; but after the change
into urea has taken place, ammonia is not revealed by this
test. Moreover, the compound formed, urea, forms salts
with acids ; it unites with hydrogen chloride, for example,
forming CON2H4.HC1. The compound from which it
is derived, ammonium cyanate, on treatment with hydro-
chloric acid, is converted into ammonium chloride, NH4C1,
and cyanic acid, which itself undergoes further change,
unnecessary to allude to here : —

NH4NCO + HC1. Aq = NH4C1. Aq + HNCO.

It is evident that here there are two compounds containing
the same elements in the same proportion by weight, and
yet having very different properties.

Faraday, in experimenting with oil-gas, produced by
heating the vapour evolved from oil at a high temperature,
attempted to condense it to a liquid by application of cold
and pressure. In this he was successful, and the compound
he obtained was identical in composition with the well-
known defiant gas, now termed ethylene, C2H4, which is
the product on heating a mixture of alcohol with concen-
trated sulphuric acid. But Faraday's product possessed a
density twice as great as that of ethylene. While ethylene
has a density approximately 14 times that of hydrogen, imply-
ing the molecular weight 28 (and C2 = 24 + H4 = 4 are to-
gether equal to 28), Faraday's gas, which is now known
as butylene, was found to have the density 28, implying a
molecular weight of 56, and involving the formula C4Hg.
Hence it appeared that two compounds could exist, of
which one might possess a molecular weight twice as great
as the other, yet of the same percentage composition ; and
to this the name polymerism was applied ; the substance

ISOMERISM AND POLYMERISM 89

of higher molecular weight is termed the polymer of that
of the simpler molecule.

It is chiefly among compounds of carbon that the pheno-
mena of isomerism and polymerism have been observed ;
although there are some well-marked cases of the latter
among compounds of other elements ; the best known is
that of NO2 and N0O4 ; the former, which exists only at
a high temperature, is a dark red gas ; the latter is almost
colourless, and is produced by cooling the former. But in
this case the two compounds, to both of which the name
nitric peroxide is applied, are very easily transformed from
one state into the other, and do not differ in their chemical
reactions. Instances of isomerism among compounds of
elements other than carbon are rare, but are not unknown.

The explanation of this curious phenomenon was sought
for in the arrangement of the atoms in the molecule. The
cases given, although they were the first noticed, are not
the simplest ; an instance will therefore be chosen from
compounds containing only the two elements carbon and
hydrogen.

One of the constituents of coal-gas is named methane,
or marsh-gas ; it escapes from the mud at the bottom of
stagnant pools when it is stirred with a stick ; its formula
is CH4. This gas, when mixed with its own volume of
chlorine, and exposed to diffuse light for some hours, ex-
changes one of the atoms of hydrogen which it contains for
an atom of chlorine ; at the same time the displaced hydro-
gen unites with another atom of chlorine, forming hydrogen
chloride, thus: CH , + C12 = CHSC1 + HCL If it be
assumed that the hydrogen in marsh-gas is in union with
the carbon, and that that union can be pictured by a stroke
or "bond" between the atoms, the formula of methane can
H

H— C— H,

be written H — C — H, and the above equation —

90 MODERN CHEMISTRY

H H

| Cl | H

H— C— H + | _ H— C— C1+ I ,
| Cl Cl

H H

on the similar assumption that a molecule of chlorine
consists of two atoms, of which one replaces hydrogen in
methane, while the other unites with that displaced hydrogen
to form H— Cl.

Now only one compound of the formula CH3C1 is known ;
hence it may be argued that the hydrogen atoms and the
chlorine atoms are symmetrically grouped round the carbon
atom in chloromethane — for so the compound is termed.

The element sodium readily reacts with chlorine ; indeed,
if hot sodium be plunged into a jar of chlorine gas, the
metal burns brightly, and a white compound of the two
is produced, which is none other than common salt, or
sodium chloride, NaCl. It is possible to withdraw the
chlorine from chloromethane by bringing the gas, dissolved
in ether (on which neither it nor sodium have any action)
in contact with chips of sodium. Another gas is produced,
of which the analysis and vapour-density show it to possess
the formula C9H6 ; and it is reasonable to suppose that it
has been produced by the union of the two groups, CH3,
left after the renewal of the atom of chlorine from CH3C1 ;
this change can be thus expressed :

H H

I : - -: |

H— C— ; Cl + Na i + i Na + Cl ;— C— H -

H H

H H

2Na— Cl + H— C— C— H.

I I
H H

ISOMERISM AND POLYMERISM 91

The new gas is called ethane. It, too, exists in only
one modification, and it is legitimate to suppose that the
atoms of hydrogen are symmetrically arranged with reference
to the two atoms of carbon.

Like methane, it can be attacked by chlorine with simi-
lar results ; the equation is : C2H6 + C12 = C2H5C1 + HC1.
And here again only one chlorethane, C2H5C1, is known ;
another argument in favour of symmetry.

If a mixture of chloromethane and chlorethane, dissolved
in ether, be treated with sodium, a third " hydrocarbon "
is formed, possessing the formula C3Hg ; it is named
propane. Its formation may be expressed thus :

H H H

-U-H .

— C— I Cl + Na j + I Na + Cl I

H H H

H H H

2Na-

L_C1 + H— C— C— C— H.

Ui

Again, only one propane is known. But if it is exposed
to the action of chlorine, two monochloropropanes are formed,
each of which has the empirical formula C3H7C1. The
chlorine atom in each isomer may be replaced by the
hydroxyl group, —OH ; the resulting products, C3H7OH,
are termed propyl and isopropyl alcohols. It is possible
to reconvert each of these compounds into its respective
chloropropane ; and it is also possible to obtain from them,
by oxidation, very different products, Propyl alcohol,
when oxidised by boiling with a solution of chromic acid,
a compound which easily loses oxygen, is converted first
into propionic aldehyde, C3H6O ; but on continued oxi-
dation the aldehyde is changed to propionic acid, C3H6O2.
On the other hand, isopropyl alcohol is oxidised by similar

MODERN CHEMISTRY

treatment to a compound called acetone, possessing the same
empirical formula as propionic aldehyde, viz., C3H6O, but
differing entirely from the latter in properties ; and if the
process of oxidation be continued, the acetone is broken up
into acetic acid and carbon dioxide, both simpler com-
pounds, containing fewer atoms of carbon than acetone.
These changes are easily represented by the following
formulae, which are termed graphic, or structural, or consti-
tutional, inasmuch as they represent to some extent the struc-
ture of each molecule.

H H H
H— C— C— C— H

Propane.
H Cl H

— C— C— C— H

H

Isochloropropane.
H H H

H— C— C— C=0

n

Propionic Aldehyde.
H OH H

H— C— C— C— H

H H H
Isopropyl Alcohol.

H H H
H— C— C— C— Cl

Hi

Chloropropane.
H H H

H— C— C— C— O— H

iii

Propyl Alcohol.
H H O— H

H— C— C— C=0

H H
Propionic Acid.

H O H
H— C— C— C— H

Acetone.

ISOMERISM AND POLYMERISM 93

Tt is evident that while propane itself is a symmetrical
substance, inasmuch as all the atoms of hydrogen are
symmetrically arranged as regards the three atoms of carbon,
as soon as an atom of hydrogen is replaced by an atom of
chlorine, two possibilities are open ; the atom of chlorine
may attach itself either to one of the end atoms of carbon,
or to the middle one. In each case a different compound
is produced ; and this is shown not only by the different
boiling-points of the two chloro-compounds, but also by
their behaviour with reagents.

The relation of propane to ethane may be conceived to
be due to the replacement of an atom of hydrogen in the
latter by the group — CH3, which is termed the methyl
group. Ethane may be viewed as methyl -methane,
H3C — CH3, and propane may also be regarded as di-
methyl-methane, H3C — CH9 — CH3, the central group
— CH2 — being taken as the methane molecule, of which
two atoms of hydrogen have been replaced by two methyl
groups. If the structural formula of propane be again
inspected, it is evident that two butanes must be possible —
one, a methyl-propane ; the other, a trimethyl- methane,
thus : —

H

H H H H TT X TT

I I I I -<r

H— C— C— C— C— H

H

H

H— C— C— C— H

Methyl- Propane. Trimethyl- Methane.

These two butanes may yield similar derivatives. It is
evident on inspection that the first will furnish two mono-
chlorobutanes, according as an atom of chlorine replaces
one of hydrogen of either of the two end atoms of carbon ;

94 MODERN CHEMISTRY

the other, if the chlorine atom is attached to either of the
middle atoms of carbon. And the second butane can also
yield two chlorobutanes, according as a chlorine atom re-
places hydrogen of one of the three — CH3 groups, or
hydrogen of the ^CH group.

Such forms of isomerism are very common, and instances
might be multiplied indefinitely.

Another form of isomerism arises when an element such
as nitrogen, which has more than one valency, is contained
in the molecule. The atom of nitrogen can be made to
occupy one of two positions as regards an atom of carbon.
A common instance of this is the isomerism of the nitriles
and the carbamines. A nitrile is a compound in which an
atom of nitrogen replaces three atoms of hydrogen, all of
which were attached to the same atom of carbon. Thus,
from ethane, C2H6, acetonitrile is derived thus : —

H H H

H— C— C— H H— C— C=N

HH

Ethane. Acetonitrile.

H-L<°

\O— H
H
Acetic Acid.

It is seen to be closely allied to acetic acid, in which
three of the atoms of hydrogen of ethane are also replaced ;
but this time two by an atom of oxygen, and the third by
the hydroxyl- group, —OH. This close connection is made
obvious by boiling the acetonitrile with dilute alkali ; the
nitrogen is evolved as ammonia, while oxygen and hydroxyl
take the place of the atom of nitrogen : —

ISOMEKISM AND POLYMERISM 95

II II

H\Q I *0 XH

H— C— C=N + H/U =H— C— Cf +N^-H

H— O— H X)H XH

H H

But an isomer of acetonitrile is also known, to which
the formula CH3N==C is ascribed ; for, on causing it
to react with water, the products are methylamine,

CH,— NH9, and formic acid, HC^ thus :—

\OH,

H H H

I I I /H .0

H— C— X= +O + H— O— H = H— C— N< +H— Cf

| | XH XOH

II H H

From this it is inferred that in the latter case the atom
of nitrogen is in direct union with the atom of carbon of the
CH3 group ; it remains united even after attack by water
in presence of acid, the latter accelerating the action.

From these instances it is evident that the position of the
elements or groups in a molecule, as well as the composition
of the molecule, determine its nature ; and this fact is even
more strikingly shown by isomerism in the group of com-
pounds related to benzene, a liquid hydrocarbon of the
formula CGH6.

It is found that when this compound is attacked by chlorine,
so that an atom of hydrogen is replaced by an atom of chlorine,
the resulting oily liquid, named chlorobenzene, having the
formula C6H5C1, exists in only one modification. But if two
atoms of hydrogen are replaced by two atoms of chlorine,
there are three compounds produced, to each of which the
empirical formula C6H4C12 may be ascribed. To what is
this isomerism due ?

The generally accepted graphic or structural formula for
benzene, first suggested by Kekul£, lately professor of
chemistry at Bonn, is —

96 MODERN CHEMISTRY

H H
C=C

/I 2\

HC6 3CH,

\5 4^
C— C
H H

in which the six atoms of carbon are arranged as a ring,
and each in combination with one atom of hydrogen. Bear-
ing in mind that a symmetrical replacement cannot produce
isomerism, it is obvious that it is a matter of indifference
whether an atom of chlorine replaces one of hydrogen at-
tached to any of the six atoms of carbon, numbered I to 6.
But when two atoms of hydrogen are replaced by two atoms
of chlorine, the case is different. The substituting atoms of
chlorine may have three distinct positions relatively to each
other ; they may replace hydrogen atoms combined with the
carbon atoms I and 2, or with I and 3, or, lastly, with
i and 4. Obviously the numbering I and 2 expresses only
any two contiguous atoms ; it is identical with 2 and 3, 3
and 4, &c. So, too, I and 3 is identical with 2 and 4, 3
and 5, &c.; and I and 4 is the same as 2 and 5 or 3 and 6.
These chlorine derivatives may be converted into numerous
others by replacing the atoms of chlorine by other atoms
or groups of atoms ; and so three series of compounds are
obtainable, all of which belong to three separate groups.
It has been found possible to determine the positions relative
to each other of the entering atoms by the following ingenious
device. Expressing, for shortness' sake, the structural
formula above given by a simple hexagon, and assuming
that carbon atoms are situated at the angles of the hexagon,
and, where not otherwise indicated, in combination with
hydrogen ; but indicating by the symbol Cl that an atom of
hydrogen has been replaced by one of chlorine, we have the
following three formulae for the three dichlorobenzenes :

ISOMERISM AND POLYMERISM 97

Cl Cl Cl Cl

Cl
Ortho; Meta; Para;

Now, if each of these compounds be separately treated with
chlorine, trichlorobenzenes are formed ; these are shown in
the lower line ; and it will be noticed that while the first
dichlorobenzene (that designated by the prefix "ortho-")
can yield two, and only two, trichlorobenzenes, the " meta-"
dichlorobenzene may yield three trichlorobenzenes, but the
" para-" dichlorobenzene only one ; or in numbers, I 2 may
yield 123 and 124 trichlorobenzenes ; I 3 may yield
I 2 3, I 3 4, and 135 trichlorobenzenes ; while I 4 di-
chlorobenzenes can yield only 124 trichlorobenzenes ; the
last is obviously identical with I 3 4, or with I 4 5, or
with 146 trichlorobenzene. In this way the actual
position of the chlorine atoms, relatively to each other, in
the three dichlorobenzenes, was established.

All these formulae, it will be noticed, are written on the
assumption that the atoms of the elements lie in a plane.
Now this assumption is exceedingly improbable. It is true
that a map represents only the length and breadth of a
country ; not the height of the mountains, unless contour
lines be made use of. And yet a map renders great service,
for we use it, allowing for this important omission. So
these structural formulas may be advantageously employed,
so long as we remember that they do not represent all the
structure. This fact must have been in the minds of many
chemists for more than ten years before J. A. LeBel
and J. H. van't Hoff pointed out independently, in 1874,
the necessity of employing formulae of three dimensions in

VOL. i. G

98 MODERN CHEMISTRY

order to explain certain cases of isomerism which were
inexplicable on the assumption that the elements were dis-
tributed on a plane surface, as in the instances already given.
This doctrine of the arrangement of the atoms of a
molecule in space of three dimensions has been termed
Stereo-chemistry.

Stereo- chemistry. — The deposit found in wine-casks,
named tartar or argol, is the potassium salt of four acids, to
which the generic name tartaric has been given. But it
has long been known that although all these acids are re-
presented by the formula C4H6O6, they differed in physical
properties. In order to understand the nature of this differ-
ence, a short explanation must be given of the nature of
polarised light.

A mineral named tourmaline is known, which is some-
times used as a gem. Its colour is usually green, and it
occurs in fairly large transparent crystals. If a slice of
this mineral be cut, it is to all appearance quite transparent,
allowing light to pass with only a slight diminution in its
intensity, as would be the case if the light were to pass
through a plate of somewhat dull greenish glass. Through
two parallel plates of tourmaline, if held in a certain
position, light still passes with scarcely diminished intensity ;
but if the one plate be slid round on its neighbour, so that
it has performed a partial revolution of 90°, the two plates
in this position are no longer transparent, but cut off light
and become practically opaque. The light, after passing
through the first plate, is said to be polarised ; and it is
possible to extinguish this polarised light by interposing
a second plate placed in a certain position.

It is now certain that the phenomena of light are to be
attributed to the propagation of waves in a fluid which
penetrates all space, even the interior of solids ; a fluid
without weight, possibly composed of particles, which,
however, must not be confused with the atoms or mole-
cules of ponderable bodies. This fluid is termed ether.
Now waves may be caused in ether by other agencies than

STEREO-CHEMISTRY 99

light. When an ordinary Leyden jar is discharged, the
process of discharging it is not instantaneous ; the spark
which passes between the knob of the electrical u tongs "
used to discharge it and the knob of the jar is merely one
of a number which pass to and fro between the knob
of the tongs and the knob of the jar until electrical
equilibrium is established. The passage of such sparks
causes waves to be propagated through the ether, waves
which differ from those of light only in their much greater
length. Such waves, whether of light or of electric dis-
turbance, are propagated at right angles to the direction of
their oscillation ; they resemble waves en the sea in this
respect, but differ inasmuch as sea-waves oscillate merely
up and down, whereas these waves of light or electric
disturbance oscillate in all possible directions at right angles
to that in which they move forward. The name applied
to such electric waves is " Hertzian waves," after their
discoverer, the late Professor Hertz of Bonn.

It has been found that the leaves of a book held with
its edge towards the source of electric waves has the
effect of polarising these waves. They are not much
diminished in intensity by their passage through the book,
and if a second book be held edgewise, with its leaves
parallel to those of the first book, the waves can still pass
on. But if the one book be turned round so that its leaves
are at right angles to those of the other, the electric waves
are blocked, and are no longer able to pass. The Hertzian
waves are so long, and are made in such a manner, that
it is possible to ascertain in which plane they are oscillat-
ing ; indeed, the experiment with the book proves this.
For it is known that if the plane of the waves coincides with
that of the pages of the book, the waves are annihilated ; they
cause electrical currents in each leaf, and are thus absorbed.
But as each leaf is separated from its neighbour by a thin
layer of air, which is a practical insulator of electricity, if
the leaves of the book are turned at right angles to the
position in which they annihilate the electric waves, these

100

MODERN CHEMISTRY

waves cannot excite currents in the leaves, because each
leaf is insulated from its neighbour, and the currents have
no scope. After passage through the book, those waves
which were originally oscillating in the plane of these
leaves are annihilated, and used up in inciting feeble electric
currents in each leaf, while those waves originally at right
angles to the plane of the book's leaves pass through.
Waves oscillating in intermediate planes, e.g. at an angle
of 45° to the plane of the leaves, are partly annihilated, and
partly pass, in proportion to the angle which they make.

The coarsely foliated structure of a book, it cannot be
doubted, is analogous to the structure of a plate of tourma-
line. It is also almost certainly foliated, but the foliations

FIG. 4.

are extremely minute, so that the influence they have in
transmitting or obscuring light waves is commensurate with
the difference in the oscillation-length of light waves and
Hertzian waves. The light which passes through tourma-
line, like the electric waves which pass through the leaves
of a book held edgewise, have this peculiarity — they oscillate
in one plane ', and resemble in that respect the waves of the
sea. They are said to be polarised.

Now it has been found that certain substances, such as
crystals of quartz or of chlorate of potassium, have the
curious property of rotating the plane of oscillation of
polarised light ; that is, light polarised by passage through
a plate of tourmaline (or by other means, for there are

STEREO-CHEMISTRY 101

more convenient plans of polarising light),. laftjct then trans-
mitted through a plate of aybtallised potassium chlorate,
is not wholly obscured when it impinges on a second plate
of tourmaline held at right angles to the first ; it is neces-
sary to turn the second tourmaline through more than a
right angle in order that total obscuration shall result. The
plane of polarisation is rotated by the chlorate crystal. If,
however, the crystal is dissolved in water, its solution has
no such property ; and it is inferred that the rotation is due,
not to any arrangement of the atoms in the molecule of
chlorate, but to the arrangement of the molecules in the
crystal. For if the rotation were due to the former cause,
it would be produced by solution as well as by the solid.
It is believed, therefore, that the molecules in a crystal of
chlorate are arranged with regard to each other like the
stones in a spiral staircase.

The case is otherwise with crystals of tartaric acid.
While one variety of tartaric acid crystal rotates the plane
of polarisation to the right, in the direction of the hands
of a watch, another variety has the opposite effect, and a
third and a fourth variety, which can be distinguished by
means to be mentioned hereafter, are without action on
polarised light. Unlike potassium chlorate, however, solu-
tions of these crystals of tartaric acid have the same effect
as the crystals themselves ; those which have a right-
handed rotatory power retain that power even when dis-
solved ; the left-handed ones remain left-handed, and the
neutral ones neutral.

In 1 86 1, Louis Pasteur, at that time assistint to
Professor Balard, of Paris, made a most important dis-
covery. It was that crystals of dextro-rotatory or right-
handed tartaric acid, which had hitherto been believed to
be regular, were all characterised by small facets, developed
only on one corner. The neutral tartaric acid, known as
racemic acid, had no such facets ; but on crystallising a
certain double salt of racemic acid containing ammonium
and sodium, Pasteur discovered that two kinds of crystals

102 MODERN CHEMISTRY

were depocited ; spine wit'i facets on 'che right upper corner
(see (tf)'Fig. 5), and some with facets on the left corner.
The facets in question are shaded in the figure. And most
singularly, the crystals, after they had been picked out and
separated from each other, when dissolved in water each
rotated the plane of polarised light, the crystals with right
facets to the right, those with left facets to the left. Up to
that time only the dextro-rotatory tartaric acid had been
known. From this Pasteur drew the inference that the
difference between the two varieties must be due to the dif-
ferent arrangement of the atoms in the molecules of the two
varieties of tartaric acid in space of three dimensions.

FIG. 5.

(a)

Pasteur also devised two other methods of separating the
two varieties of tartaric acid contained in racemic acid :
one was by preparing a salt of racemic acid with a base such
as quinicine, which itself possesses optical properties. The
salt of dextro-tartaric acid with this base is much more
soluble than that of the left-handed or Isevo-rotatory tartaric
acid ; so that the crystals which separate out on evaporating
the solution are practically pure laevo-tartrate. His other
method of effecting the separation, or, in this case, the de-
struction of the dextro-tartrate, was by allowing a solution of
racemate of ammonium to mould. The organism consumes
the dextro-tartrate, and the Isevo-tartrate remains after the
mould has been suffered to grow for a sufficient time. It

STEREO-CHEMISTRY 103

is only the dextro-compound, in fact, which serves as food
for the mould.

In 1874, LeBel and van't HofF independently pro-
pounded a theory to explain these and similar cases of
isomerism ; it is based on the conception that all molecules
occupy space of three dimensions, and that the isomerism is
caused by the different arrangement of the atoms in the
molecule. This arrangement also gives a clue to the be-
haviour of such isomers in rotating the plane of polarised
light to the left or to the right, and also indicates why
crystals composed of such molecules should develop " hemi-
hedral " facets, as those represented on only one side of a
crystal are termed.

It is not necessary to consider any particular compound
in giving a sketch of this theory ; it may be stated in general
terms.

Marsh-gas, or methane, it has already been remarked, has
the formula CH4. We have already seen that one of the
four atoms of hydrogen which it contains may be replaced
by an atom of chlorine, and the chlorine by the group CH3,
or methyl. It is possible to replace successively all four
atoms of the hydrogen of methane by atoms or groups ;
these may be all different. If we indicate such atoms
or groups by the letters P, Q, R, S, we may have
the compounds CP3Q, CP2Q2, CP2QR, and CPQRS,
as well as CP4. The stereo-chemical hypothesis is based
on the conception that the carbon atom is situated at
the central point of a pyramid built on a triangular base
(which is named a tetrahedron}, and that the elements
or groups in combination with the atom of carbon are
placed at the four corners or solid angles of the figure,
The formulae of the compounds, constructed in this
manner, would present the appearance in perspective shown
in Fig. 6.

It is evident, on inspection, that no isomerism is possible
with the molecules numbered ( I ), (2), and (3), for in each
case a mere turning of the tetrahedron into the appropriate

104

MODERN CHEMISTRY

position can place any group in any desired position relative
to the others. Thus to take (3) ; if it be supposed
that isomerism could result from the relative positions of
groups R and Q with regard to the two groups P, it is only
necessary so to turn the crystal that the position of the two
P groups is reversed, when Q will lie at the remote corner,
and R at the near corner. The case is different with
the configuration in (4). If Q and R are transposed,
as in (5), it is impossible so to place (4) that its
groups, P, Q, R, and S, correspond in position with
those in (5). In fact, (5) may be termed the " mirror-

image" of (4), for a reflection of (4) in a mirror is
identical with (5). To choose a familiar illustration, it
is not possible to convert a right hand into a left hand
except by reflecting it in a mirror. In this sense, the right
hand is a stereoisomer of the left hand. Now it is precisely
such compounds, and only such compounds, which display
isomerism of the kind described ; one variety of which
causes the plane of polarised light to be rotated to the right,
the other to the left. One of the most familiar instances
of such isomerism has been observed with the acid of sour
milk, lactic acid ; in it, the carbon atom at the centre of

STEREO-CHEMISTRY

105

the tetrahedron is coupled with four different atoms or
groups, and is termed the " asymmetric " carbon atom. P
may stand for an atom of hydrogen ; S for the hydroxyl
group, - OH ; Q represents the methyl group, — CH3 ;

~

and S the car boxy 1 group, - C

X)H,

a group common to

all the acids of carbon, and already shown on p. 94 as part
of the formula of acetic acid. The ordinary lactic acid of
sour milk is optically inactive, but its isomer extracted
from flesh-juice is lasvo-rotatory. By one of Pasteur's

methods, however, the acid from milk can be split into two
varieties, one laevo-, the other dextro-rotatory. There are,
therefore, two forms of lactic acid, both optically active ; a
mixture of the two in equal proportions has no claim to be
termed a third body, but of course it is without action on
polarised light.

It is possible for a compound to contain two or more
asymmetric carbon atoms. Such is the case with the tar-
taric acids. The structure of these acids is shown in Fig. 7.
It is supposed to be represented by two tetrahedra, placed
apex to apex, one of course being inverted. The carbon
atoms in the interior of each tetrahedron are united by one
" bond " or valency. The other three valencies of each

106 MODERN CHEMISTRY

carbon atom are employed in union with three separate
atoms or groups, P, Q, and R, and P', Q', and R'. In the
case of the tartaric acids, these are respectively an atom of
hydrogen, the hydroxyl or - OH group, and the carboxyl

sP

or — C/ group. It will be noticed that in ( I ) of

\OH

Fig. 7, if we look down on the surface P Q R, these
letters follow each other in the opposite direction to the
hands of a watch. Let us suppose that polarised light,
entering the tetrahedron from above, would experience rota-
tion in that direction ; it passes from the base of the inverted
tetrahedron to the summit. Similarly, light entering the
lower tetrahedron from below will be rotated in the same
sense, i.e. in the direction P' Q' R' ; but if it fall on the
lower tetrahedron from above, it will receive a right-handed
screw, the same in direction as the motion of the hands of a
watch. Hence the lasvo-rotation which the light acquires
by its passage from above downwards through the upper
tetrahedron is reversed and changed to a dextro-rotation
by its passage through the lower tetrahedron, seeing that it
traverses the lower one from apex to base. The one com-
pensates the other, and the molecule is inactive, or " inter-
nally compensated." But if the positions of Q' and R'
in the lower tetrahedron be interchanged, as in (2), then
polarised light, entering the tetrahedron from below, will
have a right-handed screw imparted to it ; and consequently,
if from above, a left-handed rotation, opposite to that of the
hands of a watch. It follows, then, that the left-handed
rotation which the polarised light acquires by its passage
downwards through the upper tetrahedron will be doubled
by its passage downwards through the lower one, and .
the crystal will be laevo-rotatory. Similarly, (3) shows a
dextro-rotatory arrangement. It is evident, by inverting
the figures, that the direction of rotation is not changed.
Hence we have the inactive molecules of racemic acid
(for so this variety of tartaric acid is termed) in (i),

STEREO-CHEMISTRY 107

laevo-rotatory tartaric acid in (2), and dextro-rotatory in
(3). It is, of course, not known in which order the groups
are placed to produce dextro- or Isevo-rotation, but the
idea is easily understood. A fourth variety of tartaric acid
may, of course, be prepared by mixing equal weights of
the dextro- and lasvo- varieties ; it is inactive, but it is a
mixture, and not a definite compound, and it must not
be confused with the racemic acid of (i). This fourth
variety can be separated into its constituents by Pasteur's
device of crystallising the sodium ammonium salt, and sepa-
rating by hand those crystals which have a right-handed facet
from those with a left-handed facet ; but the true racemic
acid cannot be thus resolved at ordinary temperatures ;
it must be converted into a salt of some optically isomeric
base, and heated ; on solution in water, it is now found
to consist of a mixture of dextro- and laevo-tartaric acids,
and it may be separated by crystallisation of their sodium-
ammonium salts, as before described.

The tetrahedral form appears to be characteristic of the
compounds of all tetrad elements ; for W. J. Pope has recently
obtained compounds in which the element tin is combined
with four different groups, each containing carbon and
hydrogen ; and these display optical isomerism when re-
solved by appropriate means into their stereo-chemical
isomers. The same has been shown by S. Smiles to be
true for compounds of a similar nature containing tetrad
sulphur, and this observation has been confirmed by Pope.
It will probably be found true for similar compounds of all
tetrad elements where they hold in union four different
elements or groups.

The stereo-isomerism of compounds of nitrogen has also
been proved to hold by J. A. LeBel. As nitrogen is
either a triad or a pentad, however, the tetrahedron cannot
be the fundamental figure. It is probably a pyramid erected
on a square base. LeBel made a curious discovery in this
connection : it is that the groups in combination with the
nitrogen must have at least a certain degree of complexity,

log MODERN CHEMISTRY

and a corresponding high molecular weight, otherwise such
isomers are not capable of existence. It is conjectured that
the groups combined with the nitrogen, if they are not
sufficiently large, change places, so as to form the most
stable configuration ; it is only where they are large that
such molecular rearrangement does not occur. LeBel's
work has been confirmed by Pope.

Stereo-isomerism due to double linkage. — There
is another variety of stereo-isomerism which cannot be
detected by the rotation of polarised light. It is assumed
that in such a compound as tartaric acid (see Fig. 7), the
two tetrahedra, shown connected by their apices, are free
to revolve round a vertical axis joining the two asymmetric
carbon atoms, and passing through the point of junction
of the two tetrahedra. Taking (i) of Fig. 7, if, for
example, R and R' happen to lie on a line parallel to that
axis, we may have a compound different from one in which
R and Q' should lie on that line, as in the figure. If,
however, such a configuration were to exist, it would not
be permanent, for owing to the revolution of the tetrahedra
round the vertical axis passing through their apices, the
original configuration would be produced, and R and R'
would again lie on the same vertical line. Of course this
is on the assumption that the relative positions of P', Q',
and R' are not changed, otherwise an isomeride is produced
capable of acting on polarised light.

Now, if the atoms of carbon be connected, not singly,
as in the instances in Fig. 7, but doubly, such a power of
rotation is hindered. Such a configuration is shown in
Fig. 8. The figure may be derived from one of those in
Fig. 7 by supposing R and R' to be removed by some
appropriate reagent ; the tetrahedra will then be joined
along one of the edges instead of only at the apices, and
the carbon atoms will be "doubly linked." In (i) of
Fig. 8 a double tetrahedron, like that shown in Fig. 7,
is reproduced; (2) shows the approach of the two solid
angles; in (3), R and R' are removed, giving the new

STEREO-ISOMERISM

109

configuration. Such a compound is termed " unsaturated."
By addition of such an element as bromine, the com-
pound again becomes saturated, and ( i ) is reproduced with
bromine atoms in place of R and R'.

No. (3) of Fig. 8 is reproduced in Fig. 9(1), but the
letters have been changed, so as to represent actual groups
present in two acids, named respectively fumaric and maleic

FIG. 9.

C02H

CO-,11

(1)

C02H

(4)

acids. The formula given in ( I ) is that of maleic acid.
This acid, when exposed to hydrogen bromide, HBr, com-
bines with it ; but the double linkage between the central
carbon atoms is thereby broken, and (2) is produced. The

no MODERN CHEMISTRY

upper tetrahedron is now f»ee to rotate round the axis
joining the two central carbon atoms ; and it is supposed
that rotation takes place until the position of greatest
stability is reached. In (2) we may observe that two
hydrogen atoms occupy the left corners ; a hydrogen atom
and a bromine atom occupy the solid angles projecting
towards the spectator, and two carboxyl groups are situated
on the right. By rotation of the upper tetrahedron through
an angle of 120° in the inverse direction of the hands of
a watch, H' will be vertically above the lower carboxyl
group, H" will be above the bromine atom, and the upper
carboxyl group will be above the lower atom of hydrogen,
as shown in (3). If, now, hydrogen bromide be removed
(and this is possible by treatment with caustic potash),
the configuration will be that represented in (4) ; and this,
it is believed, is the formula of fumaric acid, the other
isomer. Fumaric acid, like maleic acid, can also combine
with hydrogen bromide, but on its removal fumaric acid is
reproduced.

An acid containing two carboxyl groups often has the
property of losing the elements of water when heated, and
yielding an anhydride ; in the case before us, C2H9 ( CO OH ) 2
= C2H2(CO)2O + H2O. Now maleic acid afone has this
property ; and it is inferred that maleic acid must there-
fore possess the structure ( I ) , seeing that the carboxyl
groups are conveniently situated for losing the elements of
water, and their carbon atoms for being linked together by
an atom of oxygen. To imagine a configuration which
would pertain to an anhydride derived from (4) would be
difficult.

This kind of isomerism is also met with among the com-
pounds of nitrogen, which, it will be remembered, acts often
as a triad. For example, substances named aldoximes are
known in which nitrogen is doubly linked to carbon ; and
it is also united to a hydroxyl group. Such substances are
known in two modifications ; and it appears probable that
the two varieties possess some such configurations as :

TAUTOMERISM in

H— C— CH3 H— C— CH3

II ^d ||

N—OH HO— N

which resemble those of fumaric and maleic acids.

Tautotnerism. — One more kind of isomerism remains
to be mentioned ; a body which is said to be tautomeric
appears to show a different constitution, according to the
reagent with which it is treated. One of the earliest
instances observed of a tautomeric compound is aceto-acetate
of ethyl. Its formula is :

CH3— C— H2C— C— O— CH2— CH3,

O ' O

as shown by its reaction with caustic potash, when acetone,
CH3 — CO — CH3, is formed, the scission occurring at the
dotted line ; but the tautomeric formula

CH3— C - CH— C— CH2— CH3

A-H J

may also be ascribed to it, for it can be shown to contain

a hydroxyl group by the action of diethylamine, giving

CH3— C - CH -- C— O— CH2— CH3.

C2H5-N-C2H5 O

Other reactions point to the same possibility of rear-
rangement. Examples of tautomerism are not unknown
among compounds of elements other than carbon ; it is
probable that two sulphurous acids are capable of existence,

one possessing the formula /^\ and the other

OH,

O = S<^ . Silver sulphite appears to be a derivative

\OH
of the first, and sodium sulphite of the second of these

ii2 MODERN CHEMISTRY

forms ; and it is probable that the particular form taken
depends on the reagent which is presented to the acid.

Although the application of geometrical formulae has
proved useful in exhibiting certain cases of isomerism such
as have been considered, it is not to be supposed that
formulae to which grouping in space of three dimensions
is-4iot usually applied do not also require three-dimensional
space. Their use is not common, merely because the
spatial relations are sufficiently evident without involving
this conception. Most of us are content with a picture
as a sufficient memento of our friends ; but if we wish a
fuller presentment, a bust or a statue will give it.

CHAPTER VII
Energy

WE have seen in the last chapter that some conception can
be made regarding the form of molecules, supposing them to
occupy space of three dimensions. It is further imagined
that the atoms in the molecule, unlike those in the diagrams
given, are not quiescent, but are in motion relatively to each
other, and that the molecules themselves also change their
relative places ; both atoms and molecules contain what
is termed "energy," in virtue of this motion. When a
chemical reaction takes place, energy may be lost or gained
— lost, when atoms or molecules assume a more stable con-
dition ; gained, when the state of a resulting compound is a
less stable one than that of the substances from which it is
formed.

We must now consider what is meant by this term
" energy." Energy can exist under various forms ; for
example, when a stone falls to the ground under the influ-
ence of the earth's attraction, it loses energy after its fall ;
when a billiard-ball is set in motion, for instance, by the
tension of a spring, the spring loses and the billiard-ball
gains energy. Energy can also be communicated to sub-
stances in the form of heat when their temperature is raised ;
it may be imparted to a body in the form of an electrical
charge, and in various other ways.

We have already seen (p. 6) that Lavoisier laid down
as a maxim that matter can neither be created nor destroyed.
This same doctrine holds as regards energy ; but there is a

VOL. i. "3 H

Ii4 MODERN CHEMISTRY

difference in kind between matter and energy, for while one
form of matter, e.g. iron, cannot be changed into another
kind of matter, such as lead, one kind of energy is con-
vertible into all other kinds of energy quantitatively, so that
no loss of energy occurs during the -conversion.

An example will suffice to make this clear : In a coal-
mine the steam-engine serves to raise the coals from the pit
to the surface. The engine expends energy in overcoming
the attraction of the earth for the weight. Whence does
the engine obtain its energy ? Obviously from the expan-
sion of the steam in the cylinder, for steam (or any other
gas) loses energy in expanding. The steam is produced
by boiling water in the boiler ; water absorbs energy in
changing into steam. And this energy reaches the water
in the form of heat from the boiler fire ; the heat is pro-
duced by the combustion of coal ; and the coal, which is
the product of the decay of wood buried under the surface
of the earth, must originally have derived its energy from
the sun, the rays of which are essential to the growth of
plants.

We have here a long chain of transformations of energy ;
the chemical energy of the coal is transformed into heat,
the heat causes the expansion of the water into steam, the
steam overcomes the resistance of the piston in the cylinder,
the motion of the engine raises the weight. In all this
chain there is no loss of energy ; it is only transformed from
one kind to another. But it must not be imagined that
each kind of energy is quantitatively transformed into the
other ; for example, when the steam urges the piston forward
in the cylinder, some of the energy is lost by the friction of
the piston against the walls of the cylinder, and is converted
into heat ; and, indeed, energy tends to be degraded, that is,
to be transformed into heat-energy.

In almost all chemical reactions which take place, either
of their own accord or on rise of temperature, heat is spon-
taneously evolved. When that is the case the reaction is
terrn^i " exothermic ;" but " endothermic " reactions are

ENERGY 115

also known, in which hc^L. is absorbed. Such reactions,
however, do not take place spontaneously at ordinary tem-
peratures. All the phenomena of combustion are exothermic
reactions. We are familiar with many examples of this, as
when coal burns, when hydrogen and oxygen explode, when
gunpowder is fired — all these are examples of exothermic
reactions. The interaction of any two or more elements
which spontaneously unite to form a compound is of the
same nature as combustion.

Endothermic reactions — those in which heat is absorbed
— are usually only possible at ordinary temperatures when
an exothermic reaction proceeds at the same time. But one
point must be noticed here ; it is necessary that both the
exothermic and the endothermic reaction should be part of
the same chemical process. For example, the formation of
chloride of nitrogen by the action of chlorine upon a con-
centrated solution of ammonia is an exothermic reaction.
Chloride of nitrogen is a fearfully explosive body, detonating
with the least shock into its elements, chlorine and nitrogen ;
but while it is being formed there is formed at the same
time ammonium chloride, a substance which is produced
with great evolution of heat. These two reactions are part
of the same chemical process, and they are expressed
by the equation 4NH8+ 3C12 = NC1S+ 3NH4C1. It is
essential that both ammonium chloride and the chloride of
nitrogen should be produced by the same chemical reaction.
The combination of nitrogen and chlorine would not take
place were any other exothermic reaction unconnected with
the formation of nitrogen chloride to be going on in the
same vessel. The elements nitrogen and chlorine do not
form nitrogen-chloride when mixed, even under the influ-
ence of a high temperature, nor would they if another exo-
thermic reaction were proceeding simultaneously in contact
with the nitrogen and the chlorine. Moreover, in order
that an endothermic compound may be formed, it is not suf-
ficient that an exothermic reaction take place simultaneously ;
the heat evolved during the exothermic reaction must usually

n6 MODERN CHEMISTRY

exceed that absorbed by the formation of the endothermic
compound.

Endothermic compounds readily decompose, often with
explosion ; when they do so heat is evolved ; the compound
loses energy. This implies that the elements in the free state
or any other products of the decomposition of the endother-
mic compound contain less energy than the compound before
decomposition. On the other hand, in order to decompose
exothermic compounds, heat must be imparted to them.
The example given on p. 31 of ammonium chloride is a
case in point. It will be remembered that in order to de-
compose ammonium chloride into ammonia and hydrogen
chloride the temperature must be raised, and heat is absorbed
by the chloride ; hence its products ammonia and hydrogen
chloride in the uncombined state contain more energy than
their compound, ammonium chloride. Other substances
similar to ammonium chloride are known which dissociate
more gradually than that compound, and the characteristic
of all such dissociating bodies is this — that the higher the
temperature the less stable they are. Even water when
raised to a temperature approaching 2000° C. dissociates
partially into hydrogen and oxygen. Indeed, the rule for
all exothermic compounds is that they become less and less
stable the higher the temperature.

The opposite is the case with endothermic compounds ;
the amount of heat absorbed by the union of their con-
stituents is less the higher the temperature ; and when the
temperature surpasses a certain point peculiar to each sub-
stance the endothermic compound changes its character and
becomes exothermic. But it is not often possible to produce
endothermic compounds by bringing the elements together
at a high temperature, because in cooling down they separate
again into their ^constituents. It appears necessary to com-
municate energy to them in some form other than heat.
The formation of ozone, O3, is accomplished by passing
the silent electrical discharge through oxygen. It is pro-
bable that the disruption of the oxygen molecule O2 into

ENERGY 117

atoms is produced by the rise of temperature due to the
electric sparks, but the combination of some of these atoms
into groups of three (as well as for the most part into
groups of two) is probably to be ascribed to the energy
which they receive in the shape of electric charges. An-
other instance is that of the burning of the nitrogen of the
air when a high tension current is passed through it. Nitro-
gen and oxygen do not unite even at the highest temperature
which can be produced by the combustion of carbon, but
when a high tension current is passed through a mixture of
the two gases a true flame is produced, and combination to
nitric peroxide, NO2, takes place. This flame can be
blown out, and it can be rekindled by the help of a lighted
match. It would thus appear that endothermic compounds
can be directly formed when energy is communicated to
them electrically.

When a chemical reaction between elements, resulting in
the formation of a compound, takes place, it is not always
that compound which is formed involving the greatest ex-
penditure of energy, or in other words, the greatest evolu-
tion of heat. It is quite possible for a compound to be
produced which, when appropriately treated, will change
into a still more stable configuration. Let us take an
example : When chlorine is passed through a solution of
caustic soda, the most stable configuration of the elements is
the production of sodium chloride, water, and oxygen.
But the reaction proceeds by no means so far ; it ceases
when the products are sodium chloride, sodium hypochlorite,
and water :

2NaOH. Aq + C12 = NaCl.Aq + NaOCl.Aq + H2O.

This solution, when warmed, undergoes a further change,
and again loses energy, yielding sodium chlorate and chlo-
ride : 3NaOCl.Aq = NaClO3.Aq + fcNaCLAq. But the
change does not cease here. For on evaporating to dry-
ness, and heating it still further, the chloride is again decom-
posed into oxygen and chloride : 2NaClO3= 2NaCl + 3O2.

ii8 MODERN CHEMISTRY

This final change is also exothermic. A mechanical analogy
for such a series of transformations may be found.

Imagine a switchback railway on an incline, those
portions of the rail which usually slope upwards being
nearly level. Further imagine a carriage started over the
first incline so as to roll on to the nearly level rail ; it will
stop here ; and it will require a further push to send it over
the second incline, when it will rest on the second level
platform and require another push to cause it to roll over
the third incline on to the third level platform. These level
platforms may be taken as analogous to the intermediate
compounds before chloride of potassium is formed. As the
carriage loses energy during each fall but stops several times
before all energy is lost, so it is possible to have a number
of stages in loss of energy before the final stable stage is
reached. Such cases are by no means unfrequent ; it is not
always possible to trace their sequence as readily as in the
case given, for it is not always possible to stop at the inter-
mediate stage ; but intermediate compounds may be made
otherwise, and they obviously belong to a series similar to
that given.

It has been frequently mentioned that application of heat
is necessary in order to start a reaction ; this is analogous
to the push which must be given to the carriage in order
that it roll over the incline ; if left alone, the compound is
stable, but the imparting to it of an exceedingly small
amount of energy suffices to cause it to lose a considerable
amount of energy in passing to the next stage. From the
molecular point of view it may be imagined that the applica-
tion of heat causes a motion of the atoms within some of
the molecules of the compound ; these begin to adjust
themselves in some new form of combination, and the heat
evolved during this readjustment is imparted to those mole-
cules which have not already suffered change, and causes
them also to assume a new form of combination attended
with loss of energy.

Besides losing energy by loss of heat during the formation

ENERGY 119

of a compound, energy may be evolved in other forms. It
is well known that in order to change a liquid into gas, heat
must be imparted to it, or, if the change take place by evapo-
rating the liquid in a partial vacuum, the liquid itself will
grow cold.

Conversely, when a gas is condensed into a liquid it
parts with the energy which it previously contained. When
a solid is changed into a liquid it absorbs energy ; when a
liquid is frozen into a solid it loses energy. Now, in
many chemical reactions the products have not the same
physical state as the substances from which they are formed ;
and in this case energy is lost or gained according to cir-
cumstances. For example, when carbon dioxide is set
free by the action of an acid upon marble, a gas is pro-
duced, and the production of this gas is attended with
absorption of energy ; in order to measure the amount of
this energy it would suffice to condense that gas to liquid
and to freeze the liquid to solid and to measure the amount
of energy evolved during these transformations. It would
then be possible to ascertain the total quantity of energy
lost during the chemical change, independently of the
change of state which the products undergo on being formed.
But this is not all ; for when a gas is produced it occupies
space and displaces a certain amount of air. Imagine the
gas to be evolved at the bottom of a vertical tube, which,
of course, was originally in communication with the atmos-
phere and full of air ; the gas would expel this air from the
tube, or, in other words, raise it. Now air possesses weight,
and presses on the surface of the earth with a weight of
1.033 kilograms on each square centimeter, and the work
done by the gas in issuing into the atmosphere would
depend, in the instance given, on the sectional area of the
tube, and the height up the tube to which the carbonic
acid reached. Here energy is expended, or, as is usually
said, work is done, in raising the weight ; and in estimating
the total energy of the reaction mentioned, this work, accom-
plished against gravity, must be subtracted from the total.

120 MODERN CHEMISTRY

Many measurements of the heat evolved or absorbed during
chemical reactions have been made, chiefly by M. Berthelot
and by Professors Julius Thomsen and Stohmann. The
reaction under investigation is caused to take place in a
calorimeter, and the heat evolved or absorbed is measured
by the rise or fall of temperature of the water which it
contains. In order to measure heat evolved by combustion,
the combustion is caused to take place in a vessel enclosed
in a calorimeter ; and M. Berthelot has introduced a very
convenient piece of apparatus in which combustible sub-
stances are caused to burn in a steel bomb charged with
oxygen at a high pressure, the bomb being itself immersed
in a calorimeter.

Inasmuch as the heat evolved during chemical decom-
position of a compound must be precisely equal to that
absorbed during its formation, it is possible indirectly to
arrive at the heat of formation of many compounds of
which the component elements will not combine directly.
Let us take, for example, the case of marsh-gas or methane ;
this compound, which has the formula CH4, is made
to burn in a vessel enclosed in a calorimeter. The heat
evolved on burning 16 grams of methane with 64 grams of
oxygen is 21 1,900 calories; that evolved on burning 12
grams of carbon in 32 grams of oxygen to carbon dioxide
is 94,300 calories ; 4 grams of hydrogen when burned in
oxygen yield 1 36,800 calories. These results are generally
expressed by the following equations: —

CH4 + 4o = CO2 + 2H2O + 2 1 3,800 c.
C + 20 = CO2 -f 94,300 c.
4H + 20 = 2H2O + 1 36,800 c.

Now methane is an exothermic body ; if it were possible
to form it from its elements, carbon and hydrogen, heat
would be evolved. It is possible to imagine it decomposed
into carbon and hydrogen, when heat would be absorbed.
The heat of formation of methane, therefore, is obviously

ENERGY 121

the difference between that which is evolved when methane
is burned in oxygen, and that which is evolved when its
constituent elements, carbon and hydrogen, are burned. In
this case it is the difference between (94,300+ 136,800)
— 213,800 = 17,300. In this case the carbon is imagined
to be solid and in the form of graphite, and the hydrogen
and oxygen to be gaseous ; if the carbon were a gas, to begin
with, it would naturally give out less heat on its combustion,
because heat is necessarily absorbed in the conversion of
solid into gaseous carbon.

It might be thought, without due consideration, that a
measurement of the heat of formation of a compound in-
volves a measurement of the energy which it contains ; but
this is not so, for it is obvious that what is measured is only
the difference of the energy contained in the elements from
which it is formed and in the compound which they pro-
duce. We are as yet ignorant of the total amount of
energy contained in any element or compound.

It is possible by suitable appliances to obtain the energy
evolved during chemical combination, not as heat, but in
the form of an electric current. When two metals are
immersed in a conducting liquid or electrolyte, they at
once exhibit a difference of electric potential ; or connect-
ing by a wire the two portions of the metal which do not
dip into the liquid, that metal which has the highest electric
potential combines with one of the ions, and the electrolyte
which has thereby discharged the other ion, as already
explained on p. 36, travels through the electrolyte until it
touches the metallic plate, when it, too, is discharged and
escapes in the free state ; its charge enters the metallic
plate. The result of this action is that the chemical com-
bination of one of the metals with one of the ions of the
liquid is attended by the formation of an electric current,
and not necessarily by an evolution or absorption of heat.
Now it is possible to measure the difference of potential
between the two metals and the amount of electricity which
passes through the wire, and thus to determine the amount

122 MODERN CHEMISTRY

of energy in a form other than heat ; by this means the
loss of energy which accompanies combination has been
frequently measured.

The process, however, leads us further, for it is possible
to arrive by its help at an estimation of what has been
termed "chemical affinity;" it is of the same nature as
electric potential. The reason for this statement is as
follows : —

It has been mentioned that energy is stored up ; when
a gas is compressed the amount of energy stored will
obviously depend on the mass of the gas and on the rise
of pressure. Energy can also be stored by the raising of
a weight above the surface of the earth ; here again the
amount of energy depends on the mass or the weight
of the body raised and the distance through which it is
raised. In the case of heat, the two components of that
form of energy are temperature and a quantity analogous
to specific heat. This case requires a little further con-
sideration. The amount of heat absorbed by a piece of
any particular metal, say copper, for heat, obviously de-
pends on the mass of the copper, on the specific heat
of the copper, and on the temperature through which
it is raised ; if the mass be doubled, the amount of heat
which that copper will absorb on being raised through
the same interval of temperature will be twice the original
amount ; if the mass remain the same and the interval
of temperature be doubled, the amount of heat will again
be doubled. By choosing another metal of which the
specific heat is twice that of copper, the heat absorbed
by a weight equal to that of the piece of copper, if the
second metal is heated through the same interval of tem-
perature, will be doubled. We see, therefore, that heat
energy may also be regarded as compounded of two factors
for unit mass : —

( i ) The specific heat of the substance.

(2) The interval of temperature through which it is
raised.

ENERGY 123

Electric energy may also be regarded as compounded of
two factors —

1 i ) Electric quantity or charge.

(2) Electric potential.

Now, when a current passes through a wire, the quantity
of electricity passing depends on the potential, or, as it is
sometimes called, electric pressure, and on the diameter,
length, and material of the wire. The total energy com-
municated in the form of an electric current has, as its
factors, the quantity of electricity passing, and the potential
with which the electricity is urged along its course.

It is probable that chemical energy may also be conceived
to consist of two factors ; the one is generally called atomic
or formula weight, for chemical elements and groups enter
into and separate out of combinations in quantities pro-
portional to these numbers. At the same time it is pro-
bable that when two elements unite together they attract
each other, and that this attraction depends for its amount
on the nature of the elements which are presented to one
another ; the chemical attraction has been termed affinity.
Now it has already been explained on p. 37 that when a
current is passed through a solution of an electrolyte,
it is conveyed by the ions present in solution ; and these
ions are composed of elements, or groups of elements, each
of which carries one, two, or more electrons. It is here
evident that the quantity, of an element or group which
conveys electricity is identical with the quantity which
enters into combination ; it may be termed the equivalent,
and while the equivalent is that quantity which conveys a
unit quantity of electricity, it is also that which serves as
the unit of quantity in chemical compounds. It would
appear, therefore, that one of the factors of chemical energy
is numerically identical with one of the factors of electrical
energy, and it follows from this that the other factor must
also be proportional ; that is, a measurement of electric
potential is equivalent to a measurement of chemical potential

I24 MODERN CHEMISTRY

or affinity. Up till now, very few experiments have been
made with the object of measuring the electric potential
of systems of chemical elements ; such measurements are
much required, for it would then be possible to arrive at
an estimate of the force with which chemical elements and
groups of elements are retained in combination.

INDEX

ACETIC acid, 94

Acetonitrile, 94

Acids, 57

Affinity, chemical, 122

,, Airs," 5

Alcohols, 91

Aldoximes, in

Allotropy, 74

Alloys, 62

Analysis, 5

Anode, 35

" Aqua," 34

Argon, 73

Arsenic, density of, 70

Arsenic, allotropic, 77

Atomic weight, 13, 65, 66, 67,

68

Atoms, 9, 52
Avogadro's hypothesis, 12

BASES, 59

Benzene, 96

Bismuth, vapour-density of, 69

Boiling-point, rise of, 29, 73

Boyle's Law, 19

Bromine, density of, 69

Butylene, 88

CALORY, 14

" Calx," 4

Capacity for heat, 13

Carbides, 61

Carbon, allotropic, 74, 75

,, stereo-chemistry of, 101
Chemical-energy, 123
Chlorine, vapour-density of, 69
Classification of compounds, 56
Combining proportions, 7

Complexity of molecules, 44, 71

Concentration, 25

Conductivity, 35, 40

,, of water, 43

, , of fused sal

Constant proportions, 7

D ALTON'S Laws, 9, 22
Dephlogisticated air, 5
Diffusion, 21
Displacement, 47
Dissociation, 31, 32, 33
Double linkage, 109
Dulong and Petit's Law, 13

ELECTRIC energy, 123
Electrolysis, 35, 40
Elements, 2

,, preparation of, 45, 46,

47.

,, classification of, 49
Endothermic reactions, 115
Energy, 113
Equivalent, 15, 63, 64
Ethane, 91
Exothermic reactions, 115

FARADAY'S Law, 35
Fluorine, density of, 69
Formulae, 52
Freezing-point, lowering of, 26,

73
Fumaric acid, 109

GASES, density of, 16
Gay-Lussac's Law, IT, 20, 21
Gold, allotropic, 80
Graphic formulae, 52, 92, 93

125

126

INDEX

HALIDES, 56
Heat, atomic, 14
Heat energy, 122

,, of combustion, 120

,, of formation, 120

,, specific, 14
Helium, 73
Hydrides, 56
Hydrocarbons, 61, 92, 93
Hydrogen, density of, 69

,, discovery of, 7

Hydroxides, 59

IODINE, vapour-density of, 69
lonisation, 40
Iridium, allotropic, 80
Iron, allotropic, 80
Isomerism, 87
Isomorphism, 17

KATHODE, 35

Krypton, 73

" LAW of octaves," 50

MALEIC acid, 109

Marsh-gas, 89

Methane, 80

Migration of ions, 36, 37, 38, 39

Molecular weight, 13, 68

Monatomicity, 71

NEON, 73

Nitric peroxide, 89

Nitrides, 59

Nitrogen, stereo-chemistry of, 89

, , density of, 69
Nomenclature, 58

OSMOTIC pressure, 23, 24, 26
Oxides, 56
Oxygen, 6

,, allotropic, 77

, , density of, 69
Ozone, 77

PARAFFINS, 89, 90, 91
Partial pressures, 22

Periodic table, 50
Phases, 81, 82, 83
Phlogiston, 3
Phosphides, 60
Phosphorus, allotropic, 77

, , vapour-density of, 70

Polarised light, 98
Polymerism, 87
Propane, 91

RACEMIC acid, 101
Rhodium, allotropic, 80
Ruthenium, allotropic, 80

SELENIDES, 56

Selenium, allotropic, 79

Silicides, 6r

Silicon, allotropic, 75

Silver, allotropic, 80

Solutions, 33

Specific heat, 13, 16

Steel, 80

Stereo-chemistry, 98

Structure of compounds, 55, 56

Structural formulae, 52, 92, 93

Sulphides, 56

Sulphur, allotropic, 78. 79

,, vapour-density of, 70
phases of, 84

,, stereo-chemistry of, 107

TARTARIC acid, 101
Tautomerism, in
Tellurides, 56
Thallium, density of, 69
Tin, allotropic, 76
,, stereo-chemistry of, 107

UREA, 87

VALENCY, 51, 52
Vapour-densities, 68

WATER, phases of, 82
XENON, 73

INDEX OF NAMES

AVOGADRO, II

BACON, 4
Becher, 3
Beckmann, 28
Berthelot, 120
Berthollet, 7
Black, 5
Boyle, 4

CANNIZZARO, 15
Cavendish, 7

D ALTON, 8
Davy, 49
Deville, 31
Dulong, 10

FARADAY, 35, 87
GAY-LUSSAC, 10

HlTTORF, 36

KEKULE", 95

LAVOISIER, 5, 58
LeBel, 97
Liebig, 87
Lodge, 37

MASSON, 38
Mendele'eff, 50
Meyer, Lothar, 50
Mitscherlich, 17

NEWLANDS, 49

PASTEUR, 101
Petit, ii
Pfeffer, 24
Priestley, 5
Proust, 7

RAOULT, 28
Rey, 4
Richter, 7

SCHEELE, 5
Schonbein, 77
Stahl, 3
Stas, 64
Stohmann, 120

THOMSEN, 120
VAN'T HOFF, 26

WENZEL, 87
Wohler, 87

END OF VOL. I.

Printed by BALLANTYNE, HANSON <
Edinburgh <V London

Co.

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

AN INITIAL FINE OF 25 CENTS

WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $I.OO ON THE SEVENTH DAY
OVERDUE.

FEB

385214

30

UNIVERSITY OF CALIFORNIA LIBRARY